Apparatus for aerosol generating device

ABSTRACT

Apparatus for an aerosol generating device is described. The apparatus comprises a heating circuit for heating a heating arrangement, the heating arrangement arranged to heat an aerosol generating material to thereby generate an aerosol; a temperature sensing arrangement for sensing a temperature of the device; and a controller for controlling a supply of energy to the heating circuit. The controller is configured to: determine a characteristic that is indicative that energy is being supplied to the heating circuit over a given time period, and determine a change in temperature sensed by the temperature sensing arrangement over the given time period; and take a control action if, based on the determined characteristic and the increase in temperature sensed by the temperature sensing arrangement over the given period, the controller determines that one or more predetermined criteria that are indicative of a fault with the temperature sensing arrangement are satisfied.

PRIORITY CLAIM

The present application is a National Phase entry of PCT Application No.PCT/EP2020/056223, filed Mar. 9, 2020, which claims priority from U.S.Provisional Application No. 62/816,287, filed Mar. 11, 2019, each ofwhich is hereby fully incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to apparatus for an aerosol generatingdevice.

BACKGROUND

Smoking articles such as cigarettes, cigars, and the like, burn tobaccoduring use to create tobacco smoke. Attempts have been made to providealternatives to these articles that burn tobacco by creating productsthat release compounds without burning. Examples of such products areheating devices which release compounds by heating, but not burning, thematerial. The material may be for example tobacco or other non-tobaccoproducts, which may or may not contain nicotine.

SUMMARY

According to a first aspect of the present disclosure, there is providedapparatus for an aerosol generating device, the apparatus comprising: aheating circuit for heating a heating arrangement, the heatingarrangement arranged to heat an aerosol generating material to therebygenerate an aerosol; a temperature sensing arrangement for sensing atemperature of the device; and a controller for controlling a supply ofenergy to the heating circuit, wherein the controller is configured to:determine a characteristic that is indicative that energy is beingsupplied to the heating circuit over a given time period, and determinea change in temperature sensed by the temperature sensing arrangementover the given time period; and take a control action if, based on thedetermined characteristic and the increase in temperature sensed by thetemperature sensing arrangement over the given period, the controllerdetermines that one or more predetermined criteria that are indicativeof a fault with the temperature sensing arrangement are satisfied.

The temperature sensing arrangement may comprise a temperature sensorfor attaching to the heating element to sense a temperature of theheating element, and the one or more predetermined criteria areindicative that the temperature sensor is detached from the heatingelement.

The controller may be configured to determine a ratio of the amount ofenergy supplied to the heating circuit over the given period to theincrease in temperature sensed by the temperature sensing arrangementover the given period; and take the control action if the supply ofenergy to the heating circuit if the ratio equals or exceeds apredetermined value.

The predetermined value for the ratio may be 2000 mJ/° C. to 6000 mJ/°C., or around 4000 mJ/° C.

The control action taken by the controller if the one or morepredetermined criteria are satisfied may be to adjust the supply ofenergy to the heating circuit. For example, to stop the supply of energyto the heating circuit or to reduce the supply of energy to the heatingcircuit.

The controller may be configured to determine the predetermined criteriafor the given time period and to determine the predetermined criteriaonce for each of one or more further predetermined periods in a usagesession of the device, wherein, the predetermined periods may each havea duration of 1/80 s to 1/20 s or a duration of around 1/64 s.

The heating circuit may be an induction heating circuit, and the heatingelement may be a susceptor arrangement for being inductively heated bythe induction heating circuit, and the temperature sensing arrangementmay comprise a temperature sensor for sensing a temperature of thesusceptor arrangement.

The temperature sensor may be a thermocouple for attaching to thesusceptor arrangement.

The temperature sensing arrangement may comprise: a first temperaturesensor for measuring a first temperature in the device; and a secondtemperature sensor for measuring a second temperature in the device; andthe increase in temperature sensed by the temperature sensingarrangement over the given period may be an increase in the firsttemperature or an increase in the second temperature.

The first temperature may be a temperature of a first heating zone inthe device and the second temperature may be a temperature of a secondheating zone in the device.

The heating circuit may be configured to selectively heat the firstheating zone and the second heating zone, and the controller may beconfigured to, during the given period, activate the heating circuit toheat only one of the first heating zone and the second heating zone.

The controller may be configured to determine the predetermined criteriafor the given time period and to determine the predetermined criteriaonce for each of one or more further predetermined periods in a usagesession of the device and during each period the heating circuit may beconfigured to selectively heat only one of the first heating zone andthe second heating zone.

The increase in temperature sensed by the temperature sensingarrangement and used to determine the one or more criteria for eachperiod during the usage session may be an increase in the firsttemperature if the heating circuit is active to heat the first heatingzone during the period; and an increase in the second temperature if theheating circuit is active to heat the second heating zone during theperiod.

The heating circuit may be an induction heating circuit comprising afirst inductor coil and a second inductor coil; and the heating elementmay be a susceptor arrangement, and the first heating zone may be afirst zone of the susceptor arrangement arranged in use to be heated bythe first inductor coil and the second heating zone may be a second zoneof the susceptor arrangement arranged in use to be heated by the secondinductor coil.

The first temperature sensor may be a first thermocouple for attachingto the first zone of the susceptor arrangement and the secondtemperature sensor may be a second thermocouple for attaching to thesecond zone of the susceptor arrangement.

The first thermocouple and second thermocouple may be J-typethermocouples each comprising a constantan wire and an iron wire.

The first thermocouple may comprise a first constantan wire and thesecond thermocouple may comprise a second constantan wire and the firstthermocouple and second thermocouple may share a single iron wire.

According to a second aspect of the present disclosure there is providedan aerosol generating device comprising apparatus according to the firstaspect of the present disclosure, wherein the aerosol provision deviceis for generating an aerosol for being inhaled by a user.

The device may be a tobacco heating device, also known as aheat-not-burn device.

According to a third aspect of the present disclosure there is providedan aerosol generating system comprising an aerosol generating deviceaccording to the second aspect and an article comprising an aerosolgenerating material for being heated by the device in use to therebygenerate an aerosol.

Optionally, the aerosol generating material may comprise a tobaccomaterial.

According to a fourth aspect of the present disclosure there is provideda method for a controller of apparatus for an aerosol generating device,the apparatus comprising: a heating circuit for heating a heatingarrangement, the heating arrangement arranged to heat an aerosolgenerating material to thereby generate an aerosol; a temperaturesensing arrangement for sensing a temperature of the device; and thecontroller, wherein the controller is for controlling a supply of energyto the heating circuit; wherein the method comprises: determining acharacteristic that is indicative that energy is being supplied to theheating circuit during a given time period; determining a change intemperature sensed by the temperature sensing arrangement over the giventime period; and taking a control action if, based on the characteristicand the increase in temperature sensed by the temperature sensingarrangement over the given period, the controller determines that one ormore predetermined criteria that are indicative of a fault with thetemperature sensing arrangement are satisfied.

According to a fifth aspect of the present disclosure there is provideda set of machine-readable instructions which when executed cause themethod according to the fourth aspect to be performed.

According to a sixth aspect of the present disclosure there is provideda machine-readable medium comprising a set of instructions according tothe fifth aspect.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Subject matter hereof may be more completely understood in considerationof the following detailed description of various embodiments inconnection with the accompanying figures, in which:

FIG. 1 shows a front view of an example of an aerosol generating device;

FIG. 2 shows a front view of the aerosol generating device of FIG. 1with an outer cover removed;

FIG. 3 shows a cross-sectional view of the aerosol generating device ofFIG. 1;

FIG. 4 shows an exploded view of the aerosol generating device of FIG.2;

FIG. 5A shows a cross-sectional view of a heating assembly within anaerosol generating device;

FIG. 5B shows a close-up view of a portion of the heating assembly ofFIG. 5A;

FIG. 6 shows a schematic representation of an example induction heatingcircuit for the aerosol generating device of FIGS. 1 to 5B;

FIG. 7A shows a schematic representation of a current through aninductor of the example induction heating circuit of FIG. 6;

FIG. 7B shows a schematic representation of a voltage across a currentsense resistor of the example induction heating circuit of FIG. 6;

FIG. 8 shows a schematic representation of a voltage across a switchingarrangement of the circuit of FIG. 6;

FIG. 9 shows another schematic representation of the example inductionheating circuit for the device of FIGS. 1 to 5B;

FIGS. 10 to 13 show various parts of an example control arrangement forthe example induction heating circuit represented by previous figures;

FIG. 14 shows a flow chart representation of an example method ofcontrolling aspects of an example induction heating circuit;

FIG. 15 shows a flow chart representation of another example method ofcontrolling aspects of an example induction heating circuit;

FIG. 16 shows a schematic representation of a temperature of a susceptorand a target power to be supplied to heat the susceptor throughout anexample usage session of an aerosol generating device;

FIG. 17 shows a perspective view of the susceptor and an exampletemperature sensing arrangement for measuring a temperature of thesusceptor;

FIG. 18 shows a perspective view of the susceptor and another exampletemperature sensing arrangement for measuring the temperature of thesusceptor;

FIG. 19 shows example apparatus for providing control functions in thedevice, where the control functions provided by the example apparatus ofFIG. 19 relate to temperatures in the device; and

FIG. 20 shows a flowchart representation of a method of controllingpower supplied to heat the susceptor.

DETAILED DESCRIPTION OF THE DRAWINGS

As used herein, the term “aerosol generating material” includesmaterials that provide volatilized components upon heating, typically inthe form of an aerosol. Aerosol generating material includes anytobacco-containing material and may, for example, include one or more oftobacco, tobacco derivatives, expanded tobacco, reconstituted tobacco ortobacco substitutes. Aerosol generating material also may include other,non-tobacco, products, which, depending on the product, may or may notcontain nicotine. Aerosol generating material may for example be in theform of a solid, a liquid, a gel, a wax or the like. Aerosol generatingmaterial may for example also be a combination or a blend of materials.Aerosol generating material may also be known as “smokable material”.

Apparatuses are known that heat aerosol generating material tovolatilize at least one component of the aerosol generating material,typically to form an aerosol which can be inhaled, without burning orcombusting the aerosol generating material. Such an apparatus issometimes described as an “aerosol generating device,” an “aerosolprovision device,” a “heat-not-burn device,” a “tobacco heating productdevice,” or a “tobacco heating device” or similar. Similarly, there arealso so-called e-cigarette devices, which typically vaporize an aerosolgenerating material in the form of a liquid, which may or may notcontain nicotine. The aerosol generating material may be in the form ofor be provided as part of a rod, cartridge or cassette or the like whichcan be inserted into the apparatus. A heater for heating andvolatilizing the aerosol generating material may be provided as a“permanent” part of the apparatus.

An aerosol provision device can receive an article comprising aerosolgenerating material for heating. An “article” in this context is acomponent that includes or contains in use the aerosol generatingmaterial, which is heated to volatilize the aerosol generating material,and optionally other components in use. A user may insert the articleinto the aerosol provision device before it is heated to produce anaerosol, which the user subsequently inhales. The article may be, forexample, of a predetermined or specific size that is configured to beplaced within a heating chamber of the device which is sized to receivethe article.

FIG. 1 shows an example of an aerosol provision device 100 forgenerating aerosol from an aerosol generating medium/material. In broadoutline, the device 100 may be used to heat a replaceable article 110comprising the aerosol generating medium, to generate an aerosol orother inhalable medium which is inhaled by a user of the device 100.

The device 100 comprises a housing 102 (in the form of an outer cover)which surrounds and houses various components of the device 100. Thedevice 100 has an opening 104 in one end, through which the article 110may be inserted for heating by a heating assembly. In use, the article110 may be fully or partially inserted into the heating assembly whereit may be heated by one or more components of the heater assembly.

The device 100 of this example comprises a first end member 106 whichcomprises a lid 108 which is moveable relative to the first end member106 to close the opening 104 when no article 110 is in place. In FIG. 1,the lid 108 is shown in an open configuration, however the cap 108 maymove into a closed configuration. For example, a user may cause the lid108 to slide in the direction of arrow “A”.

The device 100 may also include a user-operable control element 112,such as a button or switch, which operates the device 100 when pressed.For example, a user may turn on the device 100 by operating the switch112.

The device 100 may also comprise an electrical component, such as asocket/port 114, which can receive a cable to charge a battery of thedevice 100. For example, the socket 114 may be a charging port, such asa USB charging port. In some examples the socket 114 may be usedadditionally or alternatively to transfer data between the device 100and another device, such as a computing device.

FIG. 2 depicts the device 100 of FIG. 1 with the outer cover 102removed. The device 100 defines a longitudinal axis 134.

As shown in FIG. 2, the first end member 106 is arranged at one end ofthe device 100 and a second end member 116 is arranged at an oppositeend of the device 100. The first and second end members 106, 116together at least partially define end surfaces of the device 100. Forexample, the bottom surface of the second end member 116 at leastpartially defines a bottom surface of the device 100. Edges of the outercover 102 may also define a portion of the end surfaces. In thisexample, the lid 108 also defines a portion of a top surface of thedevice 100. FIG. 2 also shows a second printed circuit board 138associated within the control element 112.

The end of the device closest to the opening 104 may be known as theproximal end (or mouth end) of the device 100 because, in use, it isclosest to the mouth of the user. In use, a user inserts an article 110into the opening 104, operates the user control 112 to begin heating theaerosol generating material and draws on the aerosol generated in thedevice. This causes the aerosol to flow through the device 100 along aflow path towards the proximal end of the device 100.

The other end of the device furthest away from the opening 104 may beknown as the distal end of the device 100 because, in use, it is the endfurthest away from the mouth of the user. As a user draws on the aerosolgenerated in the device, the aerosol flows away from the distal end ofthe device 100.

The device 100 further comprises a power source 118. The power source118 may be, for example, a battery, such as a rechargeable battery or anon-rechargeable battery. Examples of suitable batteries include, forexample, a lithium battery (such as a lithium-ion battery), a nickelbattery (such as a nickel-cadmium battery), and an alkaline battery. Thebattery is electrically coupled to the heating assembly to supplyelectrical power when required and under control of a controller (notshown in FIG. 2) to heat the aerosol generating material. In thisexample, the battery is connected to a central support 120 which holdsthe battery 118 in place.

The device further comprises at least one electronics module 122. Theelectronics module 122 may comprise, for example, a printed circuitboard (PCB). The PCB 122 may support at least one controller, such as aprocessor, and memory. The PCB 122 may also comprise one or moreelectrical tracks to electrically connect together various electroniccomponents of the device 100. For example, the battery terminals may beelectrically connected to the PCB 122 so that power can be distributedthroughout the device 100. The socket 114 may also be electricallycoupled to the battery via the electrical tracks.

In the example device 100, the heating assembly is an inductive heatingassembly and comprises various components to heat the aerosol generatingmaterial of the article 110 via an inductive heating process. Inductionheating is a process of heating an electrically conducting object (suchas a susceptor) by electromagnetic induction. An induction heatingassembly may comprise an inductive element, for example, one or moreinductor coils, and a device for passing a varying electric current,such as an alternating electric current, through the inductive element.The varying electric current in the inductive element produces a varyingmagnetic field. The varying magnetic field penetrates a susceptorsuitably positioned with respect to the inductive element, and generateseddy currents inside the susceptor. The susceptor has electricalresistance to the eddy currents, and hence the flow of the eddy currentsagainst this resistance causes the susceptor to be heated by Jouleheating. In cases where the susceptor comprises ferromagnetic materialsuch as iron, nickel or cobalt, heat may also be generated by magnetichysteresis losses in the susceptor, i.e. by the varying orientation ofmagnetic dipoles in the magnetic material as a result of their alignmentwith the varying magnetic field. In inductive heating, as compared toheating by conduction for example, heat is generated inside thesusceptor, allowing for rapid heating. Further, there need not be anyphysical contact between the inductive heater and the susceptor,allowing for enhanced freedom in construction and application.

The induction heating assembly of the example device 100 comprises asusceptor arrangement 132 (herein referred to as “a susceptor”), a firstinductor coil 124 and a second inductor coil 126. The first and secondinductor coils 124, 126 are made from an electrically conductingmaterial. In this example, the first and second inductor coils 124, 126are made from litz wire/cable which is wound in a helical fashion toprovide helical inductor coils 124, 126. Litz wire comprises a pluralityof individual wires which are individually insulated and are twistedtogether to form a single wire. Litz wires are designed to reduce theskin effect losses in a conductor. In the example device 100, the firstand second inductor coils 124, 126 are made from copper litz wire whichhas a substantially circular cross section. In other examples the litzwire can have other shape cross sections, such as rectangular.

The first inductor coil 124 is configured to generate a first varyingmagnetic field for heating a first section of the susceptor 132 and thesecond inductor coil 126 is configured to generate a second varyingmagnetic field for heating a second section of the susceptor 132.Herein, the first section of the susceptor 132 is referred to as thefirst susceptor zone 132 a and the second section of the susceptor 132is referred to as the second susceptor zone 132 b. In this example, thefirst inductor coil 124 is adjacent to the second inductor coil 126 in adirection along the longitudinal axis 134 of the device 100 (that is,the first and second inductor coils 124, 126 to not overlap). In thisexample the susceptor arrangement 132 comprises a single susceptorcomprising two zones, however in other examples the susceptorarrangement 132 may comprise two or more separate susceptors. In someexamples, there may be more than two heating zones. Each zone may beformed by respective parts of a single susceptor of the susceptorarrangement or by separate susceptors of the susceptor arrangement. Ends130 of the first and second inductor coils 124, 126 are connected to thePCB 122.

It will be appreciated that the first and second inductor coils 124,126, in some examples, may have at least one characteristic differentfrom each other. For example, the first inductor coil 124 may have atleast one characteristic different from the second inductor coil 126.More specifically, in one example, the first inductor coil 124 may havea different value of inductance than the second inductor coil 126. InFIG. 2, the first and second inductor coils 124, 126 are of differentlengths such that the first inductor coil 124 is wound over a smallersection of the susceptor 132 than the second inductor coil 126. Thus,the first inductor coil 124 may comprise a different number of turnsthan the second inductor coil 126 (assuming that the spacing betweenindividual turns is substantially the same). In yet another example, thefirst inductor coil 124 may be made from a different material to thesecond inductor coil 126. In some examples, the first and secondinductor coils 124, 126 may be substantially identical.

In this example, the inductor coils 124 126 are wound in the samedirection as one another. That is, both the first inductor coil 124, andthe second inductor coil 126 are left-hand helices. In another example,both inductor coils 124, 126 may be right-hand helices. In yet anotherexample (not shown), the first inductor coil 124 and the second inductorcoil 126 are wound in opposite directions. This can be useful when theinductor coils are active at different times. For example, initially,the first inductor coil 124 may be operating to heat a first section ofthe article 110, and at a later time, the second inductor coil 126 maybe operating to heat a second section of the article 110. Winding thecoils in opposite directions helps reduce the current induced in theinactive coil when used in conjunction with a particular type of controlcircuit. In one example where the coils 124, 126 are wound in differentdirections (not shown) the first inductor coil 124 may be a right-handhelix and the second inductor coil 126 may be a left-hand helix. Inanother such embodiment, the first inductor coil 124 may be a left-handhelix and the second inductor coil 126 may be a right-hand helix.

The susceptor 132 of this example is hollow and therefore defines areceptacle within which aerosol generating material is received. Forexample, the article 110 can be inserted into the susceptor 132. In thisexample the susceptor 132 is tubular, with a circular cross section.

The device 100 of FIG. 2 further comprises an insulating member 128which may be generally tubular and at least partially surround thesusceptor 132. The insulating member 128 may be constructed from anyinsulating material, such as a plastics material for example. In thisparticular example, the insulating member is constructed from polyetherether ketone (PEEK). The insulating member 128 may help insulate thevarious components of the device 100 from the heat generated in thesusceptor 132.

The insulating member 128 can also fully or partially support the firstand second inductor coils 124, 126. For example, as shown in FIG. 2, thefirst and second inductor coils 124, 126 are positioned around theinsulating member 128 and are in contact with a radially outward surfaceof the insulating member 128. In some examples the insulating member 128does not abut the first and second inductor coils 124, 126. For example,a small gap may be present between the outer surface of the insulatingmember 128 and the inner surface of the first and second inductor coils124, 126.

In a specific example, the susceptor 132, the insulating member 128, andthe first and second inductor coils 124, 126 are coaxial around acentral longitudinal axis of the susceptor 132.

FIG. 3 shows a side view of device 100 in partial cross-section. Theouter cover 102 is again not present in this example. The circularcross-sectional shape of the first and second inductor coils 124, 126 ismore clearly visible in FIG. 3.

The device 100 further comprises a support 136 which engages one end ofthe susceptor 132 to hold the susceptor 132 in place. The support 136 isconnected to the second end member 116.

The device 100 further comprises a second lid/cap 140 and a spring 142,arranged towards the distal end of the device 100. The spring 142 allowsthe second lid 140 to be opened, to provide access to the susceptor 132.A user may, for example, open the second lid 140 to clean the susceptor132 and/or the support 136.

The device 100 further comprises an expansion chamber 144 which extendsaway from a proximal end of the susceptor 132 towards the opening 104 ofthe device. Located at least partially within the expansion chamber 144is a retention clip 146 to abut and hold the article 110 when receivedwithin the device 100. The expansion chamber 144 is connected to the endmember 106.

FIG. 4 is an exploded view of the device 100 of FIG. 1, with the outercover 102 again omitted.

FIG. 5A depicts a cross section of a portion of the device 100 ofFIG. 1. FIG. 5B depicts a close-up of a region of FIG. 5A. FIGS. 5A and5B show the article 110 received within the susceptor 132, where thearticle 110 is dimensioned so that the outer surface of the article 110abuts the inner surface of the susceptor 132. This ensures that theheating is most efficient. The article 110 of this example comprisesaerosol generating material 110 a. The aerosol generating material 110 ais positioned within the susceptor 132. The article 110 may alsocomprise other components such as a filter, wrapping materials and/or acooling structure.

FIG. 5B shows that the outer surface of the susceptor 132 is spacedapart from the inner surface of the inductor coils 124, 126 by adistance 150, measured in a direction perpendicular to a longitudinalaxis 158 of the susceptor 132. In one particular example, the distance150 is about 3 mm to 4 mm, about 3 mm to 3.5 mm, or about 3.25 mm.

FIG. 5B further shows that the outer surface of the insulating member128 is spaced apart from the inner surface of the inductor coils 124,126 by a distance 152, measured in a direction perpendicular to alongitudinal axis 158 of the susceptor 132. In one particular example,the distance 152 is about 0.05 mm. In another example, the distance 152is substantially 0 mm, such that the inductor coils 124, 126 abut andtouch the insulating member 128.

In one example, the susceptor 132 has a wall thickness 154 of about0.025 mm to 1 mm, or about 0.05 mm.

In one example, the susceptor 132 has a length of about 40 mm to 60 mm,about 40 mm to 45 mm, or about 44.5 mm.

In one example, the insulating member 128 has a wall thickness 156 ofabout 0.25 mm to 2 mm, 0.25 mm to 1 mm, or about 0.5 mm.

As has been described above, the heating assembly of the example device100 is an inductive heating assembly comprising various components toheat the aerosol generating material of article 110 via an inductionheating process. In particular, the first inductor coil 124 and thesecond inductor coil 126 are used to heat respective first 132 a andsecond 132 b zones of the susceptor 132 in order to heat the aerosolgenerating material and generate an aerosol. Below, with reference toFIGS. 6 to 12, the operation of the device 100 in using the first andsecond inductor coils 124, 126 to inductively heat the susceptorarrangement 132 will be described in detail.

The inductive heating assembly of the device 100 comprises an LCcircuit. An LC circuit, has an inductance L provided by an inductionelement, and a capacitance C provided by a capacitor. In the device 100,the inductance L is provided by the first and second inductor coils 124,126 and the capacitance C is provided by a plurality of capacitors aswill be discussed below. An induction heater circuit comprising aninductance L and a capacitance C may in some cases be represented as anRLC circuit, comprising a resistance R provided by a resistor. In somecases, resistance is provided by the ohmic resistance of parts of thecircuit connecting the inductor and the capacitor, and hence the circuitneed not necessarily include a resistor as such. Such circuits mayexhibit electrical resonance, which occurs at a particular resonantfrequency when the imaginary parts of impedances or admittances ofcircuit elements cancel each other.

One example of an LC circuit is a series circuit where the inductor andcapacitor are connected in series. Another example of an LC circuit is aparallel LC circuit where the inductor and capacitor are connected inparallel. Resonance occurs in an LC circuit because the collapsingmagnetic field of the inductor generates an electric current in itswindings that charges the capacitor, while the discharging capacitorprovides an electric current that builds the magnetic field in theinductor. When a parallel LC circuit is driven at the resonantfrequency, the dynamic impedance of the circuit is at maximum (as thereactance of the inductor equals the reactance of the capacitor), andcircuit current is at a minimum. However, for a parallel LC circuit, theparallel inductor and capacitor loop acts as a current multiplier(effectively multiplying the current within the loop and thus thecurrent passing through the inductor). Allowing the RLC or LC circuit tooperate at the resonant frequency for at least some of the time whilethe circuit is in operation to heat the susceptor may therefore providefor effective and/or efficient inductive heating by providing for thegreatest value of the magnetic field penetrating the susceptor.

The LC circuit used by the device 100 to heat the susceptor 132 may makeuse of one or more transistors acting as a switching arrangement as willbe described below. A transistor is a semiconductor device for switchingelectronic signals. A transistor typically comprises at least threeterminals for connection to an electronic circuit. A field effecttransistor (FET) is a transistor in which the effect of an appliedelectric field may be used to vary the effective conductance of thetransistor. The field effect transistor may comprise a body, a sourceterminal S, a drain terminal D, and a gate terminal G. The field effecttransistor comprises an active channel comprising a semiconductorthrough which charge carriers, electrons or holes, may flow between thesource S and the drain D. The conductivity of the channel, i.e. theconductivity between the drain D and the source S terminals, is afunction of the potential difference between the gate G and source Sterminals, for example generated by a potential applied to the gateterminal G. In enhancement mode FETs, the FET may be OFF (i.e.substantially prevent current from passing therethrough) when there issubstantially zero gate G to source S voltage, and may be turned ON(i.e. substantially allow current to pass therethrough) when there is asubstantially non-zero gate G—source S voltage.

One type of transistor which may be used in circuitry of the device 100is an n-channel (or n-type) field effect transistor (n-FET). An n-FET isa field effect transistor whose channel comprises an n-typesemiconductor, where electrons are the majority carriers and holes arethe minority carriers. For example, n-type semiconductors may comprisean intrinsic semiconductor (such as silicon for example) doped withdonor impurities (such as phosphorus for example). In n-channel FETs,the drain terminal D is placed at a higher potential than the sourceterminal S (i.e. there is a positive drain-source voltage, or in otherwords a negative source-drain voltage). In order to turn an n-channelFET “on” (i.e. to allow current to pass therethrough), a switchingpotential is applied to the gate terminal G that is higher than thepotential at the source terminal S.

Another type of transistor which may be used in the device 100 is ap-channel (or p-type) field effect transistor (p-FET). A p-FET is afield effect transistor whose channel comprises a p-type semiconductor,where holes are the majority carriers and electrons are the minoritycarriers. For example, p-type semiconductors may comprise an intrinsicsemiconductor (such as silicon for example) doped with acceptorimpurities (such as boron for example). In p-channel FETs, the sourceterminal S is placed at a higher potential than the drain terminal D(i.e. there is a negative drain-source voltage, or in other words apositive source-drain voltage). In order to turn a p-channel FET “on”(i.e. to allow current to pass therethrough), a switching potential isapplied to the gate terminal G that is lower than the potential at thesource terminal S (and which may for example be higher than thepotential at the drain terminal D).

In examples, one or more of the FETs used in the device 100 may be ametal-oxide-semiconductor field effect transistor (MOSFET). A MOSFET isa field effect transistor whose gate terminal G is electricallyinsulated from the semiconductor channel by an insulating layer. In someexamples, the gate terminal G may be metal, and the insulating layer maybe an oxide (such as silicon dioxide for example), hence“metal-oxide-semiconductor”. However, in other examples, the gate may bemade from other materials than metal, such as polysilicon, and/or theinsulating layer may be made from other materials than oxide, such asother dielectric materials. Such devices are nonetheless typicallyreferred to as metal-oxide-semiconductor field effect transistors(MOSFETs), and it is to be understood that as used herein the termmetal-oxide-semiconductor field effect transistors or MOSFETs is to beinterpreted as including such devices.

A MOSFET may be an n-channel (or n-type) MOSFET where the semiconductoris n-type. The n-channel MOSFET (n-MOSFET) may be operated in the sameway as described above for the n-channel FET. As another example, aMOSFET may be a p-channel (or p-type) MOSFET, where the semiconductor isp-type. The p-channel MOSFET (p-MOSFET) may be operated in the same wayas described above for the p-channel FET. An n-MOSFET typically has alower source-drain resistance than that of a p-MOSFET. Hence in an “on”state (i.e. where current is passing therethrough), n-MOSFETs generateless heat as compared to p-MOSFETs, and hence may waste less energy inoperation than p-MOSFETs. Further, n-MOSFETs typically have shorterswitching times (i.e. a characteristic response time from changing theswitching potential provided to the gate terminal G to the MOSFETchanging whether or not current passes therethrough) as compared top-MOSFETs. This can allow for higher switching rates and improvedswitching control.

Now with reference to FIG. 6, circuitry for induction heating by thedevice 100 will be described. FIG. 6 shows a schematic representation ofa part of an induction heating circuit 600 of the aerosol generatingdevice 100. FIG. 6 shows a part of the induction heating circuit 600which comprises the first inductor coil 124 for heating the firstsusceptor zone 132 a when a varying current flows through the firstinductor coil 124. The first susceptor zone 132 a is represented in FIG.6 as having an inductive element and a resistive element to representhow the susceptor 132 couples inductively with the first inductor 124and is heated through the generation of eddy currents. It will be notedthat the device 100 additionally comprises the second inductor coil 126,which is not shown in FIG. 6. The second inductor coil 126 is also partof the induction heating circuit 600 and is controlled to heat thesecond susceptor zone 132 b as will be described below. However, for thesake of clarity, the circuit 600 will first be described with referenceto those features shown in FIG. 6.

The circuit 600 comprises a first resonator section 601, the DC voltagesupply 118 for supplying a DC voltage to the first resonator section601, as well as a control arrangement for controlling the circuit 600.The first resonator section 601 comprises the first inductor 124 and aswitching arrangement comprising a first FET 608, and the controlarrangement is configured to switch the FET 608 between a first stateand a second state in response to voltage conditions detected in thecircuit 600, as will be described in more detail below, to operate thefirst inductor 124. The circuit 600, with the exception of the susceptor132, is arranged on the PCB 122 of the device 100, with the inductorcoil 124 being connected to the PCB 122 at a first end 130 a and asecond end 130 b.

The first resonator section 601 comprises a first capacitor 606, and asecond capacitor 610, both arranged in parallel with the first inductor124 such that when the first resonator section 601 is allowed toresonate an alternating current flows between the first capacitor 606and the second capacitor 610 and through the inductor 124. As mentionedabove, the first FET 608, in this example an n-channel MOSFET, isarranged to operate as a switching arrangement in the first resonatorsection 601.

It should be noted that in other examples, the resonator section 601 maycomprise only one capacitor, for example in the position of the firstcapacitor 606, or at the position of the second capacitor 610. In otherexamples, the resonator section 601 may comprise any other number ofcapacitors, such as three or more capacitors. For example, either orboth of the first capacitor 606 and the second capacitor 610 may bereplaced by two or more capacitors arranged in parallel with oneanother. As will be well understood, the resonator section 601 has aresonant frequency which is dependent on the inductance L and thecapacitance C of the resonator section 601. The number, type andarrangement of capacitors in the resonating section 601 may be selectedbased on considerations of the power levels to be used in the circuit600 and the desired frequency of operation of the circuit 600. Forexample, it will be understood that individual capacitors and anarrangement of said capacitors can be considered to have an equivalentseries resistance (ESR) as well as a limit on the ability of saidcapacitors to handle current. Such features may be taken into accountwhen determining an arrangement of capacitors to provide the capacitancein the resonator section 601. For example, depending on desired powerlevels and frequency of operation, there may be an advantage toproviding a plurality of capacitors in parallel, to provide highercapacitance or lower ESR. In this example, the first and secondcapacitors 606, 610 are both ceramic COG capacitors each having acapacitance of around 100 nF. In other examples, other types ofcapacitors and/or capacitors with other capacitance values, e.g.capacitors with unequal capacitance values, may be used, according tothe considerations outlined in this paragraph.

The first resonator section 601 is supplied a DC voltage by the DCvoltage supply 118, which is, for example, as described above, a voltagesupplied by a battery. As shown in FIG. 6, the DC voltage supply 118comprises a positive terminal 118 a and a negative terminal 118 b. Inone example, the DC voltage supply 118 supplies a DC voltage of around4.2V to the first resonator section 601. In other examples, the DCvoltage supply 118 may supply a voltage of 2 to 10V, or around 3 to 5V,for example.

A controller 1001 is configured to control operation of the circuit 600.The controller 1001 may comprise a micro-controller, e.g. amicro-processing unit (MPU), comprising a plurality of inputs andoutputs. In one example, the controller 1001 is an STM32L051C8T6 modelMPU. In some examples, the DC voltage supply 118 provided to the circuit600 is provided by an output from the controller 1001 which itselfreceives power from a battery or other power source.

The positive terminal 118 a of the DC voltage source 118 is electricallyconnected to a first node 600A. In an example, the DC voltage source 118is connected to the node 600A via the controller 1001 which receivespower from the DC voltage source 118 and supplies the voltage suppliedby the DC voltage source to components of the device, including thecircuit 600. The first node 600A is electrically connected to a firstend 606 a of the first capacitor 606 and to the first end 130 a of thefirst inductor 124. The second end 130 b of the first inductor 124 iselectrically connected to a second node 600B, which in FIG. 6 isrepresented at two electrically equivalent points in the circuitdiagram. The second node 600B is electrically connected to a drainterminal 608D of the FET 608. In this example, the second node 600B isalso electrically connected to a first end 610 a of the second capacitor610. Continuing around the circuit, the source terminal 608S of thefirst FET 608 is electrically connected to a third node 600C. The thirdnode 600C is electrically connected to ground 616, and in this exampleto a second end 610 b of the second capacitor 610. The third node 600Cis electrically connected via a current sense resistor 615 to a fourthnode 600D, and the fourth node 600D is electrically connected to thenegative terminal 118 b of the DC voltage source 118, which, as with thepositive terminal, in an example is supplied via the controller 1001.

It should be noted that in examples where the second capacitor 610 isnot present, the third node 600C may have only three electricalconnections: to the first FET source terminal 608S, to ground 616 and tothe current sense resistor 615.

As mentioned above, the first FET 608 acts a switching arrangement inthe first resonator section 601. The first FET 608 is configurablebetween a first state, i.e. an ‘ON’ state and a second state, i.e. an‘OFF’ state. As will be well understood by those skilled in the art whenan n-channel FET is in an OFF state (i.e. when the appropriate controlvoltage is not applied to its gate) it effectively acts as a diode. InFIG. 6, the diode functionality that the first FET 608 exhibits when inits OFF state is represented by a first diode 608 a. That is, when theFET 608 is in the OFF state the first diode 608 a acts to largelyprevent current flowing from the drain terminal 608D to the sourceterminal 608S but allows current to flow from the source terminal 608Sto the drain terminal 608D if the diode 608 a is appropriately forwardbiased. An n-channel FET is in an ON state when an appropriate controlvoltage is applied to its gate so that a conductive path exists betweenits drain D and source S. As such, when the first FET 608 is in the ONstate, it acts like a closed switch in the first resonator section 601.

As mentioned above, the circuit 600 may be considered to comprise afirst resonator section 601 and an additional control arrangement. Thecontrol arrangement comprises a comparator 618, a zero-voltage detector621, and a flip-flop 622, and is configured to detect voltage conditionswithin the first resonator section 601 and to control the first FET 608in response to the detected voltage conditions. This control of thefirst FET 608 by the control arrangement will now be described in moredetail.

At the second node 600B there is electrically connected the zero-voltagedetector 621, which is configured to detect a voltage condition, i.e. avoltage of at or near 0V with respect to a ground voltage, at a point inthe circuit 600 to which the zero-voltage detector 621 is connected. Thezero-voltage detector 621 is configured to output a signal to controlswitching of the state of the FET 608. That is, the zero-voltagedetector 621 is configured to output a signal to the flip-flop 622. Theflip-flop 622 is an electrical circuit which is configurable between twostable states. The flip-flop 622 is electrically connected to a firstgate driver 623 which is configured to provide a voltage to the firstFET gate terminal 608G dependent on the state of the flip-flop. That is,the first gate driver 623 is configured to provide an appropriatevoltage to the first FET gate terminal 608G to switch the FET 608 to theON state when the flip-flop is in one state, but is configured not toprovide a voltage appropriate for maintaining the FET 608 in the ONstate when the flip-flop 622 is in the other state. For example, thefirst gate driver 623 may be configured to provide an appropriategate-source voltage to the first FET gate 608G to switch the FET 608 ONwhen the flip-flop 622 is in a state ‘1’, and the first gate driver 623may be configured not to provide the gate-source voltage when theflip-flop 622 is in state ‘0’. The state of the flip-flop means 622therefore controls whether the first FET 608 is on or off.

In this example, the zero-voltage detector 621 and the first gate driver623 of the control arrangement are configured to receive respectivesignals 1011, 1021 from the controller 1001, by which signals thecontroller 1001 can initiate and control operation of the circuit 600,as will be discussed in more detail below.

At the fourth node 600D, there is electrically connected a controlvoltage line 619. The control voltage line 619 is electrically connectedto a fifth node 600E via a resistor 617 a and the fifth node 600E iselectrically connected to the voltage comparator 618—hereinaftercomparator 618. The fifth node 600E is electrically connected to apositive terminal of the comparator 618. A negative terminal of thecomparator 618 is connected to ground 616. In this example, thecomparator 618 is configured to output a signal based on a comparison ofthe voltage at the fifth node 600E to ground voltage. The output signalof the comparator 618 is sent to the flip-flop 622. A control voltage1031 is supplied, in this example from the controller 1001, to thecontrol voltage line 619 via a second resistor 617 b.

As mentioned above, the comparator 618 is electrically connected toprovide an output to the flip-flop 622. The flip-flop 622 is configuredsuch that an output signal from the comparator 618 can change the stateof the flip-flop 622, and thereby cause the first driver 623 to changethe state of the first FET 608.

The functioning of the example circuit 600 will now be described in moredetail in the context of the first resonator section 601 being activatedby the controller 1001 such that the first inductor coil 124 is operatedto heat the first susceptor zone 132 a.

To begin, the first FET 608 is configured in the OFF state, and is thusacting as a diode 608 a, preventing current flowing through the inductor124. The controller 1001 initiates the operation of the circuit 600 toheat the first susceptor zone 132 a by causing the FET 608 to switchfrom the OFF state to the ON state. In this example the controllerinitiates operation of the circuit 600 by providing a START signal 1011to the zero-voltage detector 621. The flip-flop 622 is thereby caused tochange states and cause the first gate driver 623 to provide a signal tothe FET gate terminal 608G to thereby switch the FET to the ON state.

Once the FET 608 is switched to the ON state, what may be referred to asa self-oscillating heating cycle of the circuit 600 begins. The FET 608,now being in the ON state, acts as a closed switch allowing a DC currentto begin flowing from the DC voltage source positive terminal 118 athrough the first inductor 124 and returning to the DC voltage sourcenegative terminal 118 b via the current sense resistor 615. The firstinductor 124 opposes this initial increase in current, as is well-known,generating a back electromotive force (EMF) as understood via Faraday'sand Lenz's laws. In the ON state, the voltage between the drain terminal608D and the source terminal 608S is substantially zero.

FIG. 7A shows a schematic graphical representation of the currentflowing through the first inductor 124 against time t starting from whenthe FET 608 is switched on, at time to. From time to, a DC currentbegins to build up in the inductor 124 from zero at a rate which isdependent on an inductance L1 of the inductor 124, a DC resistance ofthe circuit 600 and the DC supply voltage. In one example the currentsense resistor 615 has a resistance of around 2 mΩ, while the inductor124 has a DC resistance of, 2 to 15 mΩ, or 4 to 10 mΩ or in this examplearound 5.2 mΩ. This build-up of current in the inductor corresponds tothe inductor 124 storing magnetic energy, and the amount of magneticenergy which can be stored by the inductor 124 is dependent on itsinductance L1, as will be well understood.

FIG. 7B shows a simplified representation of the voltage across thecurrent sense resistor 615 against time t, again from the time to whenthe FET 608 is turned on. Shortly after the FET 608 is turned on, avoltage develops across the inductor 124, this being the back EMFgenerated by the inductor 124 as the inductor opposes the increase incurrent. At this time, therefore, the voltage across the current senseresistor 615 as shown in FIG. 7B is small, since almost all of thevoltage difference provided by the DC supply 118 drops across theinductor 124. Then, as the current through the inductor 124 increasesand the back EMF of the inductor 124 decays, the voltage across thecurrent sense resistor 615 increases. This is seen as the development ofa negative voltage across the current sense resistor 615, as shown inFIG. 7B. That is, the voltage across the current sense resistor 615becomes increasingly negative with the length of time that the FET 608is on.

Since the increasingly negative voltage across the current senseresistor 615 corresponds with the increasing current through theinductor 124, the magnitude of the voltage across the current senseresistor 615 is indicative of the current flowing through the inductor124. While the FET 608 remains on, the current through the inductor 124and the voltage across the current sense resistor 615 increasesubstantially linearly towards respective maximum values I_(max),V_(max) (which are dependent on the DC voltage supplied by DC supply 118and the DC resistance of the circuit 600) with a time constant dependenton the inductance L1 and on the DC resistance of the circuit 600. Itshould be noted that as the current through the inductor 124 is varyingafter time to some inductive heating of the susceptor 132 may occurwhile the DC current through the first inductor 124 builds up.

The circuit 600 is configured such that the amount of energy which isstored in the first inductor 124 in the time during which the FET 608 isswitched on, is determined by the control arrangement and can becontrolled by the controller 1001. That is, the controller 1001 controlsan amount of DC current (and thus an amount of magnetic energy) allowedto build up in the inductor 124, as will now be described.

As described above, the control voltage 1031 is applied to the controlvoltage line 619. In this example, the control voltage 1031 is apositive voltage and the voltage input to the positive terminal of thecomparator 618 (i.e. the voltage at the fifth node 600E) at any one timeis dependent on the value of control voltage 1031 and the voltage at thefourth node 600D. When the negative voltage across the current senseresistor 615 reaches a particular value, it cancels, at the fifth node600E, the positive control voltage 1031 and gives a voltage of 0V (i.e.ground voltage) at the fifth node 600E. In this example, the resistor617 a has a resistance of 2 kΩ. The resistor 617 b represents aneffective resistance to the controller 1001 of 70 kΩ. The voltage at thefifth node 600E reaches 0V when the negative voltage across the currentsense resistor 615 has the same magnitude as the control voltage 1031.

The comparator 618 is configured to compare the voltage at its positiveterminal to the voltage of ground 616, connected to its negativeterminal, and output a signal as a result. In one example the comparatoris a standard component FAN156, as may be obtained fromOn-Semiconductor. Accordingly, when the voltage at fifth node 600Ereaches 0V, the comparator 618 receives a 0V signal at its positiveterminal, and the result of the comparison by the comparator 618 is thatthe voltage at the positive terminal is equal to the voltage at thenegative terminal. The comparator 618 consequently outputs a signal tothe flip-flop 622 and causes the FET 608 to be switched off. As such,switching off of the FET 608 is dependent on a voltage conditiondetected in the circuit 600. Namely, in this example, when thecomparator 618 detects by comparison of the voltage across its terminalsthat a negative voltage across the current sense resistor 615 hasreached the same magnitude as the control voltage 1031, which occurs attime t₁, the FET 608 is switched off. In FIG. 7A, the DC current flowingthrough the inductor 124 at time t₁ when the FET 608 is switched off islabelled I₁.

When the FET 608 is turned off, at time t₁, the FET 608 switches fromacting like a closed switch to acting like a diode 608 a in theresonator section 601, and for the purposes of supply from the DC supply118 effectively acting like an open switch. At time t₁ the path of theDC current through the inductor 124 to ground 616 is interrupted by theFET 608. This triggers the current flowing in the first inductor 124 todrop off (this is not shown in FIG. 7A), and the inductor 124 opposesthis change in current by generating an induced voltage. Accordingly,current begins oscillating back and forth between the inductor 124 andthe capacitors 606, 608 at the resonant frequency of the first resonatorsection 601.

Similarly, the voltage across the inductor 124 and thereby between thefirst FET drain 608D and source 608S terminals begins to oscillate atthe resonant frequency of the first resonator section 601. As thecurrent through and voltage across the inductor 124 begin to oscillate,the susceptor 132 is inductively heated. Switching the FET 608 to theOFF state, therefore acts to release the magnetic energy stored in theinductor 124 at time t₁ to heat the susceptor 132.

FIG. 8 shows a trace 800 of the voltage across the first FET 608,starting from the FET 608 being in the ON state from time t₀ to t₁. Overthe time illustrated in FIG. 8 the first FET 608 is turned off and ontwice.

The voltage trace 800 comprises a first section 800 a between times toand t₁ when the first FET 608 is ON, and a second section 800 b to 800 dwhen the first FET 608 is switched off. At 800 e the FET 608 is switchedon again, and a third section 800 f which is equivalent to the firstsection 800 a begins while the first FET 608 remains on and theabove-described process of building up of DC current through theinductor 124 repeats. FIG. 8 also shows a fourth section 800 g when thefirst FET 608 is switched off again to allow oscillation of the voltageacross the FET 608, and a fifth section 800 h when the first FET 608 issubsequently switched on again.

The voltage across the first FET 608 is zero when the first FET 608 ison in sections 800 a, 800 f and 800 h. When the first FET 608 is turnedoff as indicated by section 800 b to 800 d and also by section 800 g,the first inductor 124 uses the energy stored in its magnetic field(which magnetic field was the result of the DC current built up when thefirst FET 608 was on) to induce a voltage that opposes a drop in thecurrent flowing through the first inductor 124 as a result of the firstFET 608 being off. The voltage induced in the first inductor 124 causesa corresponding variation in voltage across the first FET 608. Duringthis variation in voltage, the first inductor 124 and the capacitors606, 610 begin to resonate with each other with a sinusoidal waveform.The voltage shown by voltage trace 800 initially increases (see forexample 800 b) as the induced voltage in the first inductor 124increases to oppose a drop in current due to the first FET 608 beingoff, reaches a peak (see for example 800 c) and then, as the energystored in the magnetic field of the first inductor 124 diminishes,decreases back to zero (see for example 800 d).

The varying voltage 800 b to 800 d and 800 g produces a correspondingvarying current (not shown) and, since during the off time of the firstFET 608, the capacitors 606, 610 and the first inductor 124 act as aresonant LC circuit, the total impedance of the combination of the firstinductor 124 and capacitors 606, 610 is at a minimum during this time.It will therefore be understood that the maximum magnitude of thevarying current flowing through the first inductor 124 will berelatively large. This relatively large varying current accordinglycauses a relatively large varying magnetic field in the first inductor124 which causes the susceptor 132 to generate heat. The time periodover which the voltage across the first FET 608 varies as indicated bysection 800 b to 800 d and by section 800 g in this example depends onthe resonant frequency of the first resonator section 601.

Referring now to FIG. 6 and FIG. 8, the circuit 600 is configured suchthat when the first FET 608 is off and the voltage across the first FET608 decreases back towards 0V, the zero-voltage detector 621 detectsthis voltage condition and outputs a signal to the flip-flop 622 whichcauses the first FET 608 to be switched back to the ON state. That is,in response to this voltage condition detected within the firstresonator section 601, the FET 608 is switched from the OFF state to theON state. The zero-voltage detector 621 may be considered to detect avoltage condition indicative that a given proportion of a cycle ofcurrent oscillation between the inductive element and the capacitiveelement has been completed since the FET 608 was switched off. That is,the zero-voltage detector 621 detects that a half-cycle of current (andvoltage) oscillation at the resonant frequency of the first resonatorsection 601 has been completed by the zero-voltage detector 621detecting that the voltage across the FET 608 has returned to 0V ornearly 0V.

In some examples, the zero-voltage detector 621 may detect when thevoltage across the first FET 608 has returned to at or below a voltagelevel 801 and as such may output a signal to cause switching of thestate of the FET 608 before the voltage across the FET 608 reachesexactly 0V. As is illustrated by FIG. 8, the operation of thezero-voltage detector 621 curtails oscillations of the voltage in theresonator section 601 after one half-cycle and thus results in asubstantially half-sine wave voltage profile across the first FET 608.Further details of the operation of the zero-voltage detector 621 willbe described below with reference to FIG. 9.

When the first FET 608 is switched back on, at point 800 e, a DC currentdriven by the DC source 118 again builds up through the first inductor124. The first inductor 124 may then again store energy in the form of amagnetic field to be released when the first FET 608 is next switchedoff to initiate resonance within the first resonator section 601. As thefirst FET 608 is repeatedly switched on and off in this way, the abovedescribed process is continuously repeated to heat the susceptor 132.

It should be noted that the above described building up of currentthrough the inductor 124 described with reference to FIGS. 7A and 7Boccurs both when the FET 608 is turned on initially in response to aSTART signal 1011 from the controller 1001 and when the FET 608 isswitched on subsequently by a zero-voltage condition detected by thezero-voltage detector 621. In the first instance, in response to theSTART signal 1011, the current in the inductor 124 builds upsubstantially linearly from 0. In the second instance, when the FET 608is turned back on in response to a detected zero voltage condition atpoint 800 e, some excess current is circulating in the circuit 600 (e.g.from previous cycles of switching on and off of the FET 608). As the FET608 is turned back on following the detection of a zero-voltagecondition, the recirculating current produces an initial negativecurrent through the FET 608. Then, while the FET 608 remains on, thecurrent through the FET 608 and inductor 124 builds up, substantiallylinearly, from the initial negative current value produced by therecirculating current. As the current through the inductor 124 buildsup, the voltage across the current sense resistor 615 correspondinglybecomes increasingly negative, in the manner described above.

In examples, switching on and off of the FET 608 may occur at afrequency of around 100 kHz to 2 MHz, or around 500 kHz to 1 MHz, oraround 300 kHz. The frequency at which the switching on and off of theFET 608 occurs is dependent upon the inductance L, the capacitance C,the DC supply voltage supplied by the supply 618 and further upon adegree to which current continues recirculating through the resonatorsection 601 and the loading effect of the susceptor 132. For example,where the DC supply voltage equals 3.6V, the inductance of the inductor124 is 140 nH, and the capacitance of the resonator section 601 is 100nF, the time for which the FET 608 remains on may be around 2700 ns andthe time for a half-cycle of oscillation to complete when the FET 608 isoff may be around 675 ns. These values correspond to a power of around20 W being supplied from the DC voltage supply 118 to the resonatorsection 601. The above value of the time for which the FET 608 remainson is affected by the amount of current which recirculates in thecircuit, since as described above, this recirculating current causes aninitial negative current through the inductor upon switching on of theFET 608. It should also be noted that the time for the current to buildup to the value which causes switching off of the FET 608 is also atleast in part dependent on the resistance of the inductor 124, however,this has a relatively minor effect on the time when compared to theeffect of the inductance of the resonator section 601. The time for ahalf-cycle of oscillation to complete (of in this example 675 ns) isdependent on the resonant frequency of the resonator section 601 whichis affected not only by the values of inductance and capacitance of theinductor 124 and capacitors 606, 610 respectively, but also by theeffective resistance provided by loading the inductor 124 with thesusceptor 132.

Thus far, the circuit 600 has been described in terms of its operationto heat the susceptor 132 by one inductor, the first inductor 124, andthus only a part of the circuit 600 used by the device 100 has beendescribed. However, as described above, the device 100 also comprises asecond inductor 126 for heating the second zone 132 b of the susceptor132. FIG. 9 shows the circuit 600 comprising the second inductor 126 inaddition to the first inductor 124.

As shown in FIG. 9, in addition to the features described with referenceto FIGS. 6 to 8, the circuit 600 comprises a second resonator section701 comprising the second inductor coil 126, a third capacitor 706, afourth capacitor 710 and a second FET 708, having a drain terminal 708D,a source terminal 708S, and a gate terminal 708G. Additionally, thecircuit 600 comprises a second gate driver 723 configured to provide agate-source voltage to the second FET gate terminal 708G. The controller1001 is not shown in FIG. 9 but the controller 1001 controls the circuit600 in the manner described with reference to FIGS. 6 to 8 andadditionally is configured to provide a control signal 1012 to thesecond gate driver 723. Some reference numerals of features of thecircuit 600 already described with reference to FIG. 6 have been omittedfrom FIG. 9 for the sake of clarity.

As described above, the first inductor 124 is arranged to heat the firstzone 132 a of the susceptor 132 and the second inductor 126 is arrangedto heat the second zone 132 b of the susceptor 132. The second inductor126, third and fourth capacitors 706, 710, and second FET 708 arearranged to form the second resonator section 701, in the same manner asthe first inductor 124, first and second capacitors 606, 610, and firstFET 608 are arranged to form the first resonator section 601. In oneexample, the third and fourth capacitors 706, 710 are also COGcapacitors and may have a capacitance of around 100 nF. The secondinductor 126 in one example has a DC resistance of around 8 mΩ. Whenactive, the second resonator section 701 operates to heat the susceptor132 in an equivalent manner as described above for the first resonatorsection 601 and description of this will not be repeated here.

It will be appreciated that the value of the DC resistance of theinductors 124, 126 will have an effect on the efficiency of the circuit600, since a higher DC resistance will result in higher resistive lossesin the inductor 124, 126 and as such it may be desirable to minimizeinductor DC resistance, for example by changing the number of windings,or the cross-section of the inductors 124, 126. Furthermore, it will beappreciated that an AC resistance of the inductor 124 is increasedcompared to the DC resistance due to the skin effect. As such, the useof litz wire in examples provides for reducing the skin effect, andthereby reducing AC resistance and associated resistive losses from theinductors 124, 126. To give an example, where the first inductor 124 hasa DC resistance of around 5 mΩ and the second inductor 126 has a DCresistance of around 8 mΩ, and the circuit operates at around 300 kHz,the particular arrangement of litz wire forming the coils results ineffective resistances for the inductors 124, 126 of around 1.14 timestheir DC resistance values.

A node 700A in the second resonator section 701 is equivalent to thefirst node 600A of the first resonator section 601 and is electricallyconnected to the first node 600A and thereby to the positive terminal118 a of the DC supply 118. A node 700C is at the equivalent position inthe second resonator section 701 as is the third node 600C of the firstresonator section 601 and the node 700C is similarly connected to ground616.

It is important to note that the circuit 600 is configured to beoperated by the controller 1001 such that only one of the resonatorsections 601, 701 is active at any one time. Examples of this operationwill be described in more detail below.

During the activation of one of the resonator sections 601, 701, thezero-voltage detector 621 is configured to detect a zero-voltagecondition in the active resonator section 601, 701 and thus controlswitching of the respective FET 608, 708 of the active resonator section601, 701. The zero-voltage detector 621 controls when the respective FET608, 708 of the active resonator section 601, 701, is switched back on(such as at point 800 e), as will now be described in more detail, withreference to FIGS. 8 to 10.

In the circuit 600, the zero-voltage detector 621 is configured todetect a zero-voltage condition at the second node 600B of the firstresonator section 601 or at the equivalent node 700B of the secondresonator section 701. When one of the first resonator section 601 andsecond resonator section 701 is active, the zero-voltage detector 621detects each time the respective FET 608, 708 has been switched off,that the voltage across that FET 608, 708 has returned to zero (e.g.point 800 e in FIG. 8) or, is close to zero e.g. below a level 801. Inresponse to the zero-voltage detector 621 making this detection, asignal is output to change the state of the flip-flop 622. Therespective gate driver 623 which is in operation then outputs agate-source voltage to switch the respective FET back to the ON state.

A first small signal diode 725 connects the zero-voltage detector 621 tothe first resonator section second node 600B and a second small signaldiode 726 connects the zero-voltage detector 621 to the equivalent node700B of the second resonator section 701. Specifically, anodes of thefirst small signal diode 725 and second small signal diodes areconnected to the zero-voltage detector 621 input via a common node 701Bwhile cathodes of the diodes 725, 726 are connected respectively to thenodes 600B, 700B.

The operation of the zero-voltage detector 621, in a particular example,will now be described with reference to FIG. 10, which shows thezero-voltage detector 621 and the flip-flop 622. In FIG. 10, thecomponents which make up the zero-voltage detector 621 are enclosed by adotted line box. The node 701B connected to the anodes of the first andsecond small signal diodes 725, 726 is shown. The start signal 1011 fromthe controller 1001 to the zero-voltage detector 621 can also be seen inFIG. 13.

The zero-voltage detector 621 in this example comprises an inverter gateU103 having an input 2 from the node 701B and an output 4 connected toan input of the flip-flop 622. The inverter gate U103 is powered byconnections 5 and 3 and a capacitor C108 isolates the connection 5 fromground. A logic power supply 621 a of, in this example, 2.5V is appliedto the input 5 and via a pull-up resistor R111 to the input 2 of theinverter gate U103. The logic power supply 621 a is in this examplesupplied from the controller 1001. The inverter gate U103 is configuredto act as an OR gate for the START signal 1011 and a zero-voltagedetection signal from the node 701B. That is, the inverter gate U103 isconfigured to receive a logic low signal in the form of the START signal1011 from the controller 1001 to initiate operation of the circuit 600a. The START signal 1011 may be provided by on “open drain” signal pinof the controller 1001. The inverter gate U103 is also configured toreceive a logic low signal from the node 701B when one of the first andsecond signal diodes 725, 726 is forward biased due to one of the nodes600B, 700B being at or near zero volts, as will be explained below. Wheneither or both of these logic low signals is received by the invertergate input 2 the inverter gate U103 inverts the received signal andoutputs a logic-high signal to the flip-flop 622.

When the first inductor 124 is being operated to heat the susceptor 132,the second FET 708 remains off. When the second FET 708 remains off, thesecond small signal diode 726 has either no bias or is reverse biaseddepending on the voltages at the logic power source and the DC supply118, that is, the voltage at a cathode end (nearest the node 700B) ofthe second small signal diode 726 is either substantially the same as orhigher than the voltage at an anode end (nearest the zero-voltagedetector 621) of the second small signal diode 726.

During operation of the first resonator section 601, when the first FET608 is off and the voltage across it varies as indicated by 800 b-d ofFIG. 8, the first small signal diode 725 is reverse biased. At the endof this variation in voltage, when the voltage reaches 0V as indicatedby 800 e, or is close to 0V (e.g. at or below level 801), the firstsmall signal diode 725 becomes forward biased. Accordingly, when thefirst small signal diode 725 is forward biased at 800 e, the signalprovided to the input 2 of the inverter gate U103 becomes a logic lowsignal since a voltage drop is produced from the logic signal 621 aacross the resistor R111. As such, once this logic low signal isinverted by the inverter gate U103, a logic high signal is provided atthe output 4 of the inverter gate U103.

Although in the above description the functioning of the zero-voltagedetector 621 is described in relation to controlling switching of thefirst FET 608, it will be understood that the zero-voltage detector 621functions in the same way, using the second small signal diode 726instead of the first small signal diode 725, to control the second FET708.

Still with reference to FIG. 10, the flip-flop 622 comprises a clockinput CLK, a reset input/RST, and an output Q. The flip-flop 622 alsocomprises further inputs D and VCC for supplying power, in this examplethe flip-flop receives the same 2.5V logic power supply 621 a from thecontroller 1001 as the inverter gate U103 receives. The clock input CLKis connected to the output 4 of the inverter gate U103 and is configuredto receive a signal therefrom. When the output 4 of the inverter gateU103 switches from logic-low to logic-high (due to the input 2 ofinverter gate U103 receiving a detected zero-voltage condition or aSTART signal 1011 as described above) the clock input CLK of theflip-flop 622 receives a logic-high rising edge signal which “clocks”the flip-flop 622 and makes the state of the flip-flop output Q high.The flip-flop 622 comprises a further input/RST configured to receive asignal from the output of the comparator 618, by which the comparator618 can change the state of the flip-flop 621 to cause the flip-flopoutput Q to be low. The flip-flop output Q is connected to the first andsecond gate drivers 623, 723 and on receiving a high output from theflip-flop output Q, whichever one of the gate drivers 623, 723 is active(due to receiving a signal 1021, 1022 as described above) provides agate driver signal to its respective FET 608, 708.

In one particular example, the flip-flop 622 may switch at half of thevoltage of the logic power source 621 a, that is, at 1.25V in thisexample. This means that the forward bias voltage of the first smallsignal diode 725 and the voltage at the first FET drain 608D must sum to1.25V in order that the first FET 608 is switched on. In this exampletherefore, the first FET 608 is switched on when its drain 608D is at0.55V rather than at exactly 0V. It should be noted that ideally,switching may occur at 0V across the FET 608 for maximum efficiency.This zero-voltage switching advantageously prevents the first FET 608from discharging the capacitors 606, 610 and thereby wasting energystored in said capacitors 606, 610.

FIG. 11 shows in more detail the first and second gate drivers 623, 723and their connection to the gates 608G, 708G of their respective FETs608, 708. Each of the gate drivers 623, 723 has an input IN which isconfigured to receive a signal dependent on the heater activationsignals 1021, 1022 supplied from the controller 1001. Additionally, thesignals received by the inputs IN of the gate drivers 623, 723 aredependent on whether the signal output by the flip-flop output Q ishigh. The inputs IN are connected to the output Q of the flip-flop 622via respective resistors R125, R128, which in this example each have avalue of 2 kΩ.

The gate drivers 623, 723 each have two further inputs VDD and XREFwherein each input VDD receives a 6V power supply from the controller1001 and XREF receives a 2.5V logic voltage, which in this example isthe same logic voltage supplied by the controller 1001 to the flip-flop622 and inverter gate U103. The inputs VDD of each of the first andsecond gate drivers 623, 723 are connected to a 6V supply voltage andthe inputs VDD are connected to ground via two buffering capacitorsC120, C121. The gate drivers 623, 723 also each have a terminal GNDconnected to ground wherein the terminals VDD and GND act to supplypower to the gate drivers 623, 723. In this example, the capacitorsC120, C121 each have a value of IpF. The gate drivers 623, 723 areconfigured to output gate drive voltages from respective outputs OUT.The outputs OUT of the gate drivers 623, 723 are connected respectivelyto FET gates 608G, 708G via resistors R114, R115, which in this exampleeach have a resistance of 4.999.

Each gate driver 623, 723 is configured to receive a signal at its inputIN to cause the gate driver to be activated only while a logic-highsignal is provided from the flip-flop output Q and a heater activationsignal 1021, 1022 is received from the controller 1001. An “open-drain”signal pin may be provided on the controller 1001 which is configured toprovide the signals 1021, 1022.

In examples, initiation of the circuit 600 for heating by one of theresonator sections 601, 701 proceeds by the controller 1001 firstinitiating the desired one of the gate drivers 623, 723 by a respectiveone of the heater initiation signals 1021, 1022. The controller 1001then supplies the START signal 1011 to the zero-voltage detector 621.The duration of the START signal 1011 should be shorter than the periodof half a cycle of oscillation by the active resonator section 601, 701(this period may be referred to as the “resonant fly-back period”). Thisallows the circuit to properly begin self-oscillating in response to adetected zero-voltage condition. In another example, the order the STARTsignal 1011 and respective heater enable signal 1021, 1022 may bereversed such that the START signal 1011 is first applied to set theflip-flop Q output to high, and one of the heater initiation signal1021, 1022 then applied to begin the self-oscillating of the resonatorsection 601, 701 corresponding to heater to which the signal 1021, 1022is supplied.

To continue with describing in more detail the control arrangement forcontrolling the circuit 600, FIG. 12 shows a portion of the controlarrangement comprising the comparator 618 and associated components. InFIG. 12, the positive terminal 118 a of the DC power supply 118 is shownconnecting to a node 1500A which is connected to nodes 600A, 700A of thefirst and second resonator sections 601, 701 respectively. The negativeterminal 118 b of the DC power supply is connected to a node 1500B whichis equivalent to the node 600D shown in FIG. 6. The node 1500B connectsto ground 616 via the current sense resistor 615. Between the nodes1500A and 1500B an arrangement of capacitors C111, C112, C115 and C116,each in this example having a capacitance of 100 μF, are connected inparallel, providing buffering between nodes 1500A and 1500B.

FIG. 12 shows in more detail components associated with the functioningof the comparator 618 for detecting that the current through the activeinductor 124 or 126 has reached a given level. As described withreference to earlier figures, the comparator 618 acts to compare avoltage indicative of an amount of DC current flowing in the activeinductor (124 or 126) to a control voltage 1031 originating from thecontroller 1001. The comparator 618 receives power via an input 6 whichis connected via a 100Ω resistor R116 to a 2.5V logic power signal, inthis example supplied by the controller 1001 and the same logic signalas the signal 621 a received by the flip-flop 622. The comparator powerinput 6 is connected to ground via a 10 nF capacitor C119. A furtherterminal 2 of the comparator 618 is connected directly to ground.

In some examples, the controller 1001 is a micro-processing unitcomprising a timer (not shown) for generating a signal which producesthe control voltage 1031. In this example, the control voltage 1031 isproduced by a pulse-width modulated signal PWM_DAC generated by thecontroller 1001. The timer of the controller 1001 generates apulse-width modulated square waveform, with, for example, a magnitude ofaround 2.5V and a frequency of around 20 kHz and having a particularduty cycle. The pulse-width modulated signal PWM_DAC is filtered by 10nF capacitors C127 and C128, and by two 49.9 kΩ resistors R121, R123 anda 10 kΩ resistor R124 to provide a substantially constant controlvoltage 1031 at the frequency at which the controller 1001 controls thecontrol voltage 1031 (of, e.g., around 64 Hz in examples). To adjust thecontrol voltage 1031, the controller 1001 in examples is configured toadjust the duty cycle of the pulse-width modulated signal PWM_DACapplied to the circuit 600. As such, the components positioned betweenthe input PWM_DAC and the positive terminal of the comparator 618effectively provide for the control voltage 1031 to be produced by apulse-wave modulated signal and for the control voltage 1031 magnitudeto be adjusted by adjusting the duty cycle of this pulse wave modulatedsignal. The control voltage line 619 shown in FIGS. 6 and 9 may thus bereplaced by these components. However, in other examples, the controlvoltage 1031 may be produced by a substantially constant voltagesupplied, for example, by the controller 1001. In such examples, some orall of the components for shown in FIG. 12 for filtering the signalPWM_DAC may not be present.

The node 1500B which is input to the comparator 618 positive input is,as mentioned above, equivalent to the node 600D of the circuit 600. Itcan be seen from FIG. 12 that, as described with reference to FIG. 6,the node 1500B is connected via the resistor 617 a to the positive inputof the comparator 618. As such, the operation of the comparator 618 isas described above: to receive an input at its positive terminal whichis dependent on the control voltage 1031 and the voltage across thecurrent sense resistor 615. When the voltage at the positive terminal ofthe comparator 618 reaches ground voltage, a signal/FF RST is output,via a resistor R118, to the flip-flop 622 input/RST to change the stateof the flip-flop 622 and thereby switch the active FET 608/708 off.

FIG. 13 shows further components for a particular example of the controlarrangement for the circuit 600. The components shown in FIG. 13 definecurrent sense apparatus 1300 for providing a signal I_SENSE indicativeof an amount of current drawn from the DC voltage supply 118 duringoperation of the circuit 600. From this signal, the controller 1001 maydetermine a current drawn from the voltage supply 118, and may use thisalong with a value of the voltage supplied by the DC voltage supply 118to determine a value for a power supplied to the circuit 600. In someexamples, as will be described below, a determined value of power may beused by the controller 1001 for controlling the circuit 600.

An input 1301 to the current sense apparatus 1300 is provided via theresistor R120 shown in FIG. 12. The input is therefore connected to thenode 1500B via the resistor R120 and receives a voltage indicative ofthe voltage across the current sense resistor 615. The current senseapparatus 1300 operates as a low-side current sensing apparatus for thecircuit 600. In that regard, the current sense apparatus 1300 comprisesan op-amp U110 running on a voltage of 3.8V supplied to an input 5 ofthe op-amp U110 (component type TS507) set up for low-side currentsensing using the current sense resistor 615, as will be wellunderstood. A transistor U109 with built-in bias resistor (componenttype RN4986) acts to switch a 2.5V supplied by the controller 1001 up tothe 3.8V supply for the op-amp U110. The power supply line from thetransistor component U109 is connected to ground via a 10 nF capacitorC132. Further, a 1 kΩ resistor R130 is connected between the positiveinput of the op-amp U110 and ground and a 412 kΩ resistor R129 isconnected between the 2.5V input from the controller 1001 and thepositive input of the comparator U110. The negative terminal of theop-amp U110 receives a voltage dependent on the voltage across thecurrent sense resistor 615. A resistor R131 and capacitor C133 in seriesprovide filtering of the voltage signal received via the input 1301. Afurther resistor R133 (in this example having resistance of 97.6 kΩ) anda 10 nF capacitor C134 are connected in parallel between the input tothe negative terminal of the op-amp U110 and the output of the op-ampU110 such that op-amp operates in a closed-loop mode.

The position of the current sense resistor 615, which, as mentionedabove, in one example is a 2 mΩ resistor, in the circuit allows for aplurality of parameters to be measured with one current sense resistor,which may allow for good efficiency. That is, the position of thecurrent sense resistor 615 in the circuit allows measurement of: the FETpeak current, which may be used, for example, in control of theinduction heating power of the circuit; the average current out of thebattery, which may be used in monitoring discharge of the battery andsetting the induction power; and the average current into the battery,which may be used, for example, in monitoring charging of the battery.

The op-amp U110 operates to output a voltage signal I_SENSE to thecontroller 1001 which is indicative of the current through the currentsense resistor 615 and thus allows the controller 1001 to determine thecurrent drawn from the DC voltage supply 118 through the circuit 600.

It should be noted that having regard to the first and second FETs 608and 708, and the topology of the circuit 600, the phasing of the firstand second inductor coils 124 and 126 with respect to each other may bechosen such that when the first inductor coil 124 is being operated,current sufficient to cause significant heating of the susceptor 132 isprevented from flowing in the second inductor coil 126, and when thesecond inductor coil 126 is being operated, current sufficient to causesignificant heating of the susceptor 132 is prevented from flowing inthe first inductor coil 124.

As described above, the first 608 and second 708 FETs effectively act asdiodes 608 a, 708 a when switched off and so may conduct a current ifthey are forward biased (i.e. the FETs are not perfect switches).Accordingly, in examples the circuit 600 may be configured so that whenone of the first 124 and 126 inductor coils is active to heat thesusceptor 132, the voltage induced across the other inactive inductorcoil does not forward bias the intrinsic diode of the FET associatedwith that inactive inductor coil but instead reverse biases it.

The effect of the above described control arrangement being configuredto control the switching arrangements 608, 708 of the circuit 600 inresponse to detected voltage conditions is that when one of the firstresonator section 601 and the second resonator section 701 is active(i.e. its gate driver 623, 723 is activated by the controller 1001) thatresonator section “self-oscillates”, while the other section remainsinactive. That is, switching of the respective FET 608, 708 in theresonator section 601, 701 repeats at a high frequency as a firstvoltage condition (detected by the comparator 618) causes the FET to beswitched from on to off, and a second voltage condition (detected by thezero-voltage detector 621) causes the FET to be switched from off to on.

The controller 1001 is configured to control the induction heatingcircuit 600 of the device 100 such that only one of the first inductor124 and the second inductor 126 is active at any one time. Thecontroller 1001 is configured to determine at a predetermined frequencywhich of the first inductor 124 and the second inductor 126 to activate.

In examples, during usage of the device 100 the controller 1001determines at the predetermined frequency, i.e. one time for each of aplurality of predetermined time intervals, which of the first resonatorsection 601 and the second resonator section 701 to activate. In oneexample, each time the controller 1001 determines which of the firstresonator section 601 and the second resonator section 701 to activate,the controller 1001 may determine to activate that resonator section toheat the susceptor 132 for the duration of the next predeterminedinterval. That is, where the predetermined frequency (which may bereferred to as an “interrupt rate”) is 64 Hz, for example, thecontroller 1001 may determine at predetermined intervals of 1/64 s,which resonator section 601, 701 to activate for a following duration of1/64 s until the controller makes the next determination of whichresonator section 601, 701, at the end of the following 1/64 s interval.In other examples, the interrupt rate may be, for example, from 20 Hz to80 Hz, or correspondingly the predetermined intervals may be of length1/80 s to 1/20 s. In order to determine which inductor 124, 126 is to beactivated for a predetermined interval, the controller 1001 determineswhich susceptor zone 132 a, 132 b should be heated for thatpredetermined interval. In examples, the controller 1001 determineswhich susceptor zone 132 a, 132 b should be heated with reference to ameasured temperature of the susceptor zones 132 a, 132 b, as will bedescribed below.

FIG. 14 shows a flowchart representation of an example method ofdetermining which of the two resonator sections 601, 701, should beactivated for a particular predetermined interval. In this example, thecontroller 1001, determines which of the first 601 and second 701resonator sections to activate for the predetermined interval based on apresent temperature T1 of the first susceptor zone 132 a heated by thefirst inductor 124 and a present temperature T2 of a second susceptorzone 132 b heated by the second inductor 126. In an example, the presenttemperatures T1 and T2 of the first 132 a and second 132 b susceptorzones may be measured by respective thermocouples (examples of which aredescribed below) attached to each zone of the susceptor 132. Thethermocouples provide an input to the controller 1001 allowing thecontroller 1001 to determine the temperatures T1, T2. In other examples,other suitable means may be used to determine the respectivetemperatures of the susceptor zones 132 a, 132 b.

At block 1051, the controller 1001 determines a present value of thetemperature T1 and compares this to a target temperature target1 for thefirst zone 132 a arranged to be heated by the first inductor 124. Thetarget temperature target1 of the first zone 132 a has a value which mayvary throughout a usage session of the device employing the circuit 600.For example, a temperature profile may be defined for the first zonedefining values for target1 throughout a usage session of the device100.

At block 1052 the controller 1001 performs the same operation as wasperformed for the first inductor 124 at block 1051 and determineswhether the present temperature T2 of the second zone 132 b is below thetarget temperature target2 for the second zone 132 b at this time.Again, the target temperature of the second zone 132 b may be defined bya temperature profile defining values of target2 throughout a usagesession. The temperature of the second zone 132 b may, similarly to thefirst zone 132 a, be measured by any suitable means such as by athermocouple.

If the answers at block 1051 and block 1052 are both “no”, i.e. bothsusceptor zones 132 a, 132 b are presently at or above their respectivetarget temperatures target1, target2, then the controller 1001determines neither of the first and second resonator sections 601, 701should be activated for the next predetermined interval.

If the answer at block 1051 is “no” and the answer at block 1052 is“yes”, i.e. the first zone 132 a is at or above its target temperaturetarget1 but the second zone 132 b is below its target temperaturetarget2, then the controller 1001 determines that the second resonatorsection 701 should be activated to heat the second zone 132 b for thenext predetermined interval.

If the answer at block 1051 is “yes” and the answer at block 1052 is“no”, i.e. the first zone 132 a is below its target temperature target1and the second zone 132 b is at or above its target temperature target2,then the controller 1001 determines that the first resonator section 601should be activated to heat the first zone 132 a for the nextpredetermined interval.

If the answer at block 1051 is “yes” and the answer at block 1052 is“yes”, i.e. both the first 132 a and second 132 b zones are below theirrespective target temperatures target1, target2, then the controller1001 continues to block 1053. At block 1053 the controller 1001effectively acts to alternate activation of the first resonator section601 and second resonator section 701 for each predetermined intervalthat both zones 132 a, 132 b remain below their respective targettemperatures.

In some examples, in order to alternately activate the first 601 andsecond 701 resonator sections, at block 1053 the controller 1001determines if an even number of predetermined intervals has elapsedsince the start of the session. If an even number of predeterminedintervals has elapsed since the start of the session then the controller1001 determines that the first resonator section 601 should be activatedfor the next interval. If an odd number of predetermined intervals haselapsed since the start of the session then the controller 1001determines that the second resonator section 701 should be activated forthe next interval. In other examples, it should be understood, that thecontroller 1001 may instead activate the second resonator section 701when an even number of intervals has elapsed and the first resonatorsection 601 when an odd number of intervals has elapsed.

In certain examples, the circuit 600 is configured such that once one ofthe resonator sections 601, 701 is activated by receipt of a signal 1021or 1022 at one of the gate drivers 623, 624, the that resonator section601/701 continues to operate, i.e. self-oscillate, until deactivated bythe controller 1001, for example by providing a different signal to thegate driver of that resonator section 601/701. As such, upon determiningwhich of the resonator sections 601, 701 to activate during a giveninterval, the controller 1001, in order to initiate this activation maydeactivate one of the resonator sections 601, 701 which was activeduring a previous interval.

To illustrate an example of block 1053 where method 1050 shown in FIG.14 is performed with intervals of 1/64 s, if the controller 1001determines that both zones 132 a, 132 b are below their respectivetarget temperatures target1, target2 and an even number of 1/64 sintervals has elapsed since the start of the usage session of the device100, then the controller 1001 activates the first resonator section 601for the next 1/64 s interval while the second resonator section 701 isrendered inactive, which in examples requires the controller 1001deactivating the second resonator section 701. If after this nextinterval of 1/64 s both zones 132 a, 132 b remain below their respectivetarget temperatures target1, target2, then for the following 1/64 sinterval the controller 1001 activates the second resonator section 701while the first resonator section 601 is rendered inactive, which inexamples requires the controller 1001 deactivating the second resonatorsection 701. For each interval in which both zones 132 a, 132 b remainbelow their respective target temperatures this alternating betweenactivating the first 601 and second 701 resonator sections continues.

Altogether, the method 1050 has the effect that the two inductors 124,126 are never activated at the same time. Where it is determined thatboth inductors 124, 126 require activation to bring their respectivezones 132 a, 132 b to target temperature the controller 1001 alternatesthe supply of power to the inductors 124, 126 at the predeterminedfrequency to bring both zones 132 a, 132 b up to their respective targettemperature. Therefore, during at a particular point in a usage session,activation of the inductor coils 124, 126 to heat their respectivesusceptor zone 132 a, 132 b may be alternating at a particularfrequency, such as 64 Hz. It can be seen that, for example, during aperiod of a usage session comprising a plurality of intervals where thefirst zone 132 a, is substantially below its target temperature and thesecond zone 132 b is at or above its target temperature, the method 1050has the effect that power may be supplied to the first resonator section601 for close to 100% of this period. However, for a period of a usagesession comprising a plurality of intervals in which both zones 132 a,132 b are below their target temperatures, each inductor may receivepower for roughly 50% of this period.

In examples, the controller 1001 is also configured at predeterminedintervals that coincide with the predetermined intervals at which themethod 1050 is performed, to determine a power being supplied to one ofthe resonator sections 601, 701 from the DC supply 118.

As described above, with reference to FIGS. 9 to 11 in particular, inorder to control which of the first resonator section 601 and secondresonator section 701 is active at any one time, the controller 1001 aswell as transmitting a START signal 1001 to initiate operation of thecircuit 600 is configured to selectively transmit a first heateroperation signal 1011 to the first gate driver 623 to activate the firstresonator section 601 or a second heater operation signal 1012 to thesecond gate driver 723 to activate the second resonator section 701.

For example, when the controller 1001 initiates operation of the circuit600 and the controller 1001 transmits the first heater operation signal1011, the circuit 600 operates as described above to activate the firstinductor 124 to heat the first susceptor zone 132 a. When the controller1001 transmits the second heater operation signal 1012 the circuit 600operates to activate the second inductor 126 to heat the secondsusceptor zone 132 b. If the controller 1001 transmits neither of thefirst heater signal 1011 and the second heater signal 1012 then neitherinductor 124, 126 is activated and the susceptor 132 is not heated.

The controller 1001 is configured to control the power supplied from theDC voltage supply 118 to the circuit 600 for inductive heating of thesusceptor 132 based on a comparison of a measurement of power suppliedto the circuit 600 and a target power. The controller 1001 is configuredto control the power supplied to the circuit 600 by controlling theswitching arrangement of the circuit 600, i.e. by controlling switchingof the FETs 608, 708. The controller 1001 may control switching of theFETs 608, 708 by setting the control voltage 1031 which determines theDC current which is allowed to build up in the inductor 124, 126corresponding to that FET 608, 708 before the FET 608, 708 is switchedoff.

FIG. 15 shows an example method 1100 performed by the controller 1001 tocontrol the power supplied to the circuit 600. At block 1101 thecontroller 1001 determines the power P supplied from the DC supply 118to the circuit 600. For example the controller 1001 may determine anaverage of the power supplied to the circuit 600 during the previouspredetermined interval. In examples, the power P being supplied to thecircuit 600 during an interval may be determined by measurement of thevoltage across and DC current being driven through a given one of theresonator sections 601, 701. The controller 1001 may then determine theproduct of the voltage across and DC current through the given one ofthe resonator sections 601, 701 to determine the power P supplied tothat resonator section 601, 701.

In examples, the determined power P is an average power supplied fromthe DC supply 118 over the predetermined interval which may bedetermined by determining a product of the average DC voltage across theDC supply 118 and the average DC current drawn from the DC supply 118over the previous interval.

In the example device 100, the DC supply 118 is a battery which isconnected to the controller 1001, and the controller 1001 then outputsthe voltage of the DC supply 118 to the circuit 600. The controller 1001is configured to determine the DC voltage supplied by the battery 118.The current drawn from the battery 118 is determined by the operation ofthe current sense apparatus 1300. The controller 1001 determines the DCvoltage and DC current once for every 1/64 s interval. The DC voltagecan be considered to be essentially constant over this short timeperiod. However, the current is varying at a rate dependent on the rapidrate of switching on and off the circuit. As described above, this isaround 300 kHz in some examples. The current sense apparatus 1300 asdescribed above with reference to FIG. 13, outputs a signal I_SENSEwhich is filtered to remove this around 300 kHz signal. An average DCcurrent for the 1/64 s interval is therefore obtained by taking ameasurement of this filtered signal I_SENSE, and the measurement ofI_SENSE is taken just before the end of the 1/64 s interval in order toallow the signal from the filter to settle. The controller 1001 therebyobtains a DC voltage and DC current measurement for the 1/64 s intervaland can calculate a product of these values to obtain the determinedpower P. This determined power P may be considered to be an average ofthe power supplied by the DC supply 118 over the 1/64 s interval.

At block 1102 the supplied power P determined at block 1101 is comparedto a target power. Where the determined power P is an average power overthe predetermined interval, the target power is a target average powerover the same interval. In one example, the target power is a target forthe average power supplied over the predetermined interval and may havea value of between 10 and 25 W, or between 15 and 23 W, or around 20 W.In this example, the target power is a range, for example, 20-21 W or of15-25 W. The controller 1001 may accordingly at block 1102 compare thesupplied power value P determined at block 1101 to the target range anddetermine whether the supplied power is below the range, within thetarget range, or above the target range. For example, where the targetrange is 20-21 W, at block 1102 the controller 1001 determines whetherP<20 W, or 20 W≤P≤21 W, or P>21 W.

Based on the comparison of the supplied power P to the target range, thecontroller 1001 determines whether and how to adjust the power for thenext predetermined interval with the aim of bringing the actual powersupplied to the active inductor 124 or 126 during the next predeterminedinterval towards the target power range. That is, if the supplied powerP is below the target range, then the controller 1001 determines toincrease the power supplied to the circuit 600 over the nextpredetermined interval. If the supplied power P is above the targetrange, then the controller 1001 determines to decrease the powersupplied to the circuit 600 over the next predetermined interval. If thesupplied power P is below the target range, then the controller 1001determines not to adjust the power supplied to the circuit 600 over thenext predetermined interval.

Due to the configuration of the circuit 600 described above, thesupplied power P for a given predetermined interval is dependent on thevalue of the control voltage 1031 for that interval. Taking the exampleof one 1/64 s interval for which the first resonator section 601 isactive, this 1/64 s interval comprises many repeating cycles comprisingsections 800 a to 800 e of the voltage trace 800 and repeats thereof.For each cycle during the period of time t₁ to t₀ the resonator section601 is allowed to resonate, and since for this period the FET 608 isoff, no power is drawn from the DC supply 118 through the firstresonator section 601. Substantially all of the power drawn from the DCsupply 118 during the given 1/64 s interval to power the resonatorsection 601 is thus drawn during the period between to and t₁ while theinductor 124 is being “energized” with current, i.e. while the FET 608is on. The time between t₁ and to is determined by the resonantfrequency of the first resonator section 601. This resonant frequencymay remain substantially constant, at least throughout a given 1/64 sinterval (although may vary over the period of operation of the circuit600 due to dependence on coil and susceptor temperature and batteryvoltage). The length of time t₀ to t₁ is determined by the value of thecontrol voltage 1031, as well as the DC voltage supplied by the DCsupply 118 and the resistance and inductance of the first resonatorsection 601 (the same applying for the second resonator section 701).That is, for a given DC supply voltage, the control voltage 1031 setsthe current I₁ which is allowed to build up in the inductor 124 betweent₀ and t₁, but where the DC supply voltage is reduced, the time requiredto build up a given value of I₁ is increased. As such, the average powersupplied during the 1/64 s interval is dependent on the value of thecontrol voltage 1031.

In examples, therefore, in order to control the power supplied to thecircuit 600 during the next interval the controller 1001 sets the valueof the control voltage 1031 for the next interval. In examples, for agiven DC supply 118 over a predetermined interval during which one ofthe resonator sections 601, 701 is active, a larger positive value ofthe control voltage 1031 results in a larger value of power P beingdelivered to the circuit 600. Therefore, in such examples, where thecontroller 1001 determines that the supplied power P over the lastinterval was above the target range, the controller 1001 reduces thecontrol voltage 1031 for the next interval. Where the controller 1001determines that the supplied power P over the last interval was belowthe target range, the controller 1001 increases the control voltage 1031for the next interval. And, where the controller 1001 determines thatthe supplied power P over the last interval was above the target range,the controller 1001 leaves the control voltage 1031 unchanged for thenext interval.

It should be noted that in one example of the above method 1100 thepower supplied P is determined at block 1101 is a power supplied to aparticular one of the resonator sections 601, 701. For example, thepower P may be determined by measuring the voltage across the firstresonator section 601 and the DC current through the first resonatorsection 601. In such an example, it is the power P supplied to the firstresonator section 601 which is used to control the control voltage 1031.It should also be noted that for a given control voltage 1031, in someexamples, the power supplied to each of the inductors 124, 126 when therespective resonator sections 601, 701 are active may be different. Thismay be, for example, because the inductors 124, 126 have differentvalues of inductance or DC resistance, or the capacitance of the tworesonator sections 601, 701 is not equal. Therefore, in this example,during a given predetermined interval, a target power outside of thetarget power range may be supplied to the second resonator section 701but since the control voltage 1031 is controlled based on the power Psupplied to the first resonator section 601, in this example thecontroller 1001 may not adjust the control voltage 1031.

For example, for a given value of the control voltage 1031, thecontroller 1001 may determine at block 1101 that an average power of 20W was supplied to the first resonator section 601 over a given interval,with the target voltage in this example being 20-21 W. At block 1102 thecontroller 1001 determines that the supplied voltage was within thetarget range and accordingly the controller 1001 determines not toadjust the control voltage 1031. Consider that for the nextpredetermined interval, the controller 1001 determines (by the examplemethod 1050) that the second resonator section 701 and not the firstresonator section 601 is to be activated. For the given value of thecontrol voltage 1031, in this example, 22.5 W is delivered due todifferences in the electrical properties of the first 601 and second 701resonator sections. However, in this example, at block 1102 thecontroller 1001 compares the last measured value of power P delivered tothe first resonator section 601 and therefore determines at block 1103not to adjust the control voltage 1031. As such, in an example of themethod 1100 the actual power supplied to the circuit 600 may be outsideof the target range. However, this may allow for controlling the powersupplied to the inductors 124, 126 by measuring only the power Psupplied to one of the resonator sections 601, 701. This may provide asimple and useful solution to maintain the power supplied to the circuit600 to within an acceptable range if, for example, the resonatorsections 601, 701 and components thereof have roughly similar electricalproperties.

As mentioned above, in some examples, the DC supply 118 is a batterywith a voltage of around 2 to 10V, or 3 to 5V, or in one example around4.2V. In some examples, the DC voltage produced by the DC supply 118 maychange, e.g. decrease, over the time that the circuit 600 is operated.For example, where the DC voltage source 118 is a battery, the batterymay initially supply a voltage of 4.2V but the voltage supplied by thebattery may reduce as the battery depletes. After a given period,therefore, the DC voltage source 118 may supply, for example, 3.5Vinstead of an initial 4.2V.

As described above, at a given supply voltage, the value of the controlvoltage 1031 controls the amount of current which is allowed to build upin the active inductor 124/126 before the respective FET 608/708 isswitched off. Power is supplied from the DC voltage supply 118 to“energize” the active inductor 124/126 by allowing a build-up of DCcurrent when the FET 608, 708 is on. As was also described above, thetime t₁ for the current to build up to the value which causes switchingof the FET 608/708 is dependent on the DC voltage supply. Therefore, forexample, if the voltage supplied by the DC supply 118 reduces, the rateat which current builds up in the inductor coil 124 reduces, resultingin reduced power P being supplied to the circuit 600.

The example method 1100 may provide for a target power to be maintainedeven in the event that the supplied voltage from the DC supply 118changes. That is, since an actual supplied power P is determined andused to control the control voltage 1031, the controller 1001 can act tomaintain a target power by adjusting the control voltage 1031. Forexample, where the battery level has depleted, the controller 1001measures that the power P supplied to the circuit 600 at a given controlvoltage 1031 has reduced, and acts to increase the power P supplied tothe circuit by increasing the control voltage 1031. As such, a targetpower level may be maintained while a battery used to power the circuit600 depletes. This is advantageous since maintaining a target powerlevel may provide for optimal efficiency of operation of the inductionheating circuit 600. For example, maintaining a substantially constantpower supplied allows for consistent heating of the aerosolizablematerial 110 a regardless of supply voltage. Similarly, the examplemethod 1100 provides for providing a substantially constant powerregardless of other changing factors in the circuit which might affectthe amount of power delivered, such as different loading on the circuit600 being provided by the susceptor 132 when the susceptor 132temperature increases. This provides a consistently good experience forthe consumer, for example by providing a consistent time to first puff,i.e. a consistent time between the device 100 being activated and beingready to provide aerosol to be inhaled by the user.

In another example, the measured power value P upon which control of thecontrol voltage 1031 is based is changed throughout a usage session. Forexample, during a particular usage session, for a first part of theusage session (e.g. a first ˜60 s of the usage session), the temperatureprofile may be such that the first inductor 124 is primarily active,while the second inductor 126 is inactive. For this first part of theusage session it may be appropriate to base control of the controlvoltage 1031 on measurements of the power delivered to the firstresonator section 601. However, later in the session, again e.g. due tothe temperature profile for the session, it may be that the secondinductor 126 is primarily active, while the first inductor 124 is activefor less of the time. Thus, for a second part of the usage session (e.g.after ˜60 s), it may be advantageous to control the control voltage 1031based on the measurements of power delivered to the second resonatorsection 701. The controller 1001 may accordingly switch from basingcontrol of the control voltage 1031 on measurements of power supplied tothe first resonator section 601 to basing control of the control voltage1031 on measurements of power supplied to the second resonator section701. In this way, the target power may be more closely adhered tothroughout a usage session, since, for example, the control voltage 1031is being set based on a comparison of the actual power being deliveredto the active inductor 124, 126 to the target power range.

In some examples, where the controller 1001 determines at block 1103that the power should be adjusted, the controller 1001 may adjust thecontrol voltage 1031 in predetermined steps. For example, the controller1001 may be configured to adjust the control voltage 1031 by apredetermined amount per predetermined time interval. Where at block1102 the controller 1001 determines that the supplied power P was belowthe target power range the controller 1001 may increase the controlvoltage 1031 by a predetermined number of volts for the nextpredetermined interval. Conversely, where at block 1102 the controller1001 determines that the supplied power was above the target power rangethe controller 1001 may increase the control voltage 1031 by apredetermined amount for the next predetermined interval.

In the example described above with reference in particular to FIG. 12,the control voltage 1031 is produced by a pulse-wave modulated signalPWM_DAC. The signal PWM_DAC, as described above has a rectangularwaveform at 2.5V. The duty signal of the signal PWM_DAC is controllableby the controller 1001 which sets a value of 0 to 800 for the PWM_DACduty cycle, this value corresponding to a duty cycle of 0% at 0 and 100%at 800. The signal PWM_DAC when filtered provides the substantiallyconstant control voltage 1031 and therefore the settings of from 0 to800 of the duty cycle of the PWM_DAC signal provide for the controlvoltage 1031 to have a magnitude of from 0 to 2.5V. In this example, thecontroller 1031 may adjust the duty cycle setting of the PWM_DAC signalby a set amount, such as 8 out of 800, or leave the setting unchanged,for each predetermined interval. In another example, the controller 1001may provide for the control voltage 1031 to be adjusted by some othermeans, and if the controller 1001 determines that the control voltage1031 should be adjusted, the controller 1001 may adjust the controlvoltage 1031 by, e.g., 1%, or 2%, or 5% of the maximum value of thecontrol voltage 1031 for the next predetermined interval.

In some examples, when operation of the circuit 600 is initiated by thecontroller 1001, e.g. to start a use session of the device 100comprising the circuit 600, the control voltage 1031 is set to apredetermined initial value. In one example, a value of the controlvoltage 1031 (for example, a duty cycle setting of the signal PWM_DACwhich produces this value of the control voltage 1031) which correspondswith a target power level is determined during setup of the circuit 600.That is, the power delivered to the circuit 600 may be determined (e.g.measured or determined theoretically) for a number of values of thecontrol voltage 1031, for example to produce a calibration curve. Avalue of the control voltage 1031 corresponding to the target power maythen be determined. In one example, the DC supply 118 may supply 4.2Vand to achieve a target power of 20 W the controller 1001 may determinein an example calibration a value for the duty cycle of the PWM_DACsignal setting of around 344 out of 800.

In one example, the controller 1001 is configured to set the controlvoltage 1031 at an initial value which is based on this determined valueof the control voltage 1031. For example, the initial value of the dutycycle of PWM_DAC which determines the control voltage 1031 may be set athalf of the determined value corresponding to the target power. Forexample, where the duty cycle setting for the control voltage 1031 foundto correspond with the target power is 344 out of 800, the controller1001 may begin the session with the setting being set at 152 out of 800,and increase the setting by a predetermined amount with everypredetermined interval until the measured power P is within the targetrange. This may have the effect that at the start of a usage session,the power delivered is well below the target power and the powerdelivered may then ramp up (by ramping up by the controller 1001 of thecontrol voltage 1031) until it reaches the target power range. Thisinitial ramping up of the power delivered may provide for improvedsafety in operation of the circuit 600, preventing overheating of thesusceptor at the start of a session and allowing the circuit 600 torespond to the actual power supplied as determined by the controller1001.

In one example, the predetermined interval is the same predeterminedinterval as is used by the controller 1001 in the method 1050 ofdetermining which of the first 124 and second 126 inductors to activate.In one such example, as mentioned above, the predetermined intervals areof length 1/64 s. The length of the predetermined interval (orequivalently the interrupt rate) may be chosen to provide anadvantageous time interval at which the controller can monitor thecircuit and adjust parameters accordingly. For example, an interruptrate of 64 Hz, or within a range of approximately 10-100 Hz may be used.At these example interrupt rates, the controller 1001 may measureincreases in temperature of the susceptor zones at a sufficiently highrate that it may determine to stop heating by a particular inductor 124,126 before a zone 132 a, 132 b of the susceptor 132 can increase too farabove its target temperature. Similarly, examples given for theinterrupt rate may provide an advantageous frequency at which thecontrol voltage 1031 may be adjusted to allow appropriate control ofpower supplied to the inductors 124, 126 to within a safe target range.

In an example method of operation of the circuit 600, a target power foruse by the controller 1001 in controlling power delivered to the circuit600 is predetermined based on characteristics of a planned usagesession. For example, the target power range may be adjusted throughouta usage session.

FIG. 16 shows a schematic example of a temperature profile target1 for aportion of a usage session, which in this example is a targettemperature for the single susceptor zone 132 a. In this example,initially at a first part 1201 of the portion of the usage session, thefirst zone 132 a is substantially below its target temperature target1.At this first part 1201, the circuit 600 is operating to bring the firstzone 132 a up to the target temperature target1. At such an example partof the usage session, a target power P1 may have a range of values of,for example, 20-21 W. The target power during the first part 1201 of thesession may be relatively high in order to bring the susceptor 132, andtherefore the aerosolizable material 110 a, up to temperature quickly toa temperature suitable for producing aerosol for inhaling by the user.

As the usage session progresses, the first zone 132 a substantiallyreaches its target temperature target1. A second part 1202 of the usagesession may be defined beginning shortly after the first zone 132 areaches its target temperature target1. For instance, for this part 1202of the usage session, the first zone 132 a may be substantially at itstarget temperature target1, of e.g. 250° C., and may be being maintainedat the target temperature target1 according to the method 1050.Similarly, although this is not shown in FIG. 16, the second zone 132 bmay be being maintained at its own target temperature target2 by themethod 1050 (and the target temperature target2 of the second zone 132 bmay define a different temperature profile to that defined by target1).

The part 1202 in the usage session after the first zone 132 asubstantially reaches temperature target1 may be characterized in thatthe controller 1001 is operating to maintain the temperature of thefirst zone 132 a (or of both zones 132 a, 132 b) rather than to bringthe temperature of the first zone 132 a up to its target value target1,as in the first part 1201. As such, during the part 1202 of the usagesession, relatively little power may be required to be supplied to thesusceptor zone 132 a to maintain the target temperature target1, whencompared to the power required to bring the susceptor zone 132 a up tothe target temperature target1. At the second part 1202 of the usagesession, it may be advantageous to reduce the value of target power P1compared to its value in part 1201. In one example, the target powerlevel P1 may be reduced from 20-21 W in part 1201 to around 15 W duringpart 1202 of the usage session. Reducing the target power P1 in this waymay be advantageous in some examples because by using a lower level ofpower energy losses in the circuit may be reduced, and thus efficiencymay be increased.

For a third part 1203 of the usage session, the value of the targettemperature target1 is 0, i.e. the first inductor 124 is not to beactivated. At this point, the target power P1 may also be reduced to 0if the usage session has come to an end, or if the second inductor 126is still being activated, then the target power P1 may remain at anon-zero value while the second inductor 126 is activated. Accordingly,the target power may take into account the temperature profile of bothzones 132 a, 132 b at any one point in the usage session. If a part ofthe usage session, for example, requires one of the zones to besignificantly increased in temperature, then a relatively high targetpower may be appropriate. Conversely, for parts of a usage session whereneither zone 132 a, 132 b requires substantial heating, a relatively lowtarget power may be used.

As mentioned above, use of lower power levels during certain periods ofa usage session may provide advantages in that an energy saving may beachieved over the duration of a session. For example, where the targetpower level is reduced from 20-21 W in the first period to around 15 Win the second period, in some examples an energy saving of around 5-10%may be achieved due to reduced energy losses in the circuit 600 whenoperating at lower power. In one example, over the course of a typicalsession of around 260 s in length, maintaining the target power ataround 20 W for the full duration of the session may result in energyusage of around 1000 J. However, reducing the target power to around 15W upon the first zone 132 a first reaching its set temperature andmaintaining the target power level at 15 W for the remainder of asession of substantially the same length may result in an energy usageof between 900 and 950 J. In examples, almost all of the power used bythe device is due to energy supplied to heat the susceptor 132. Thepower usage of electrical components other than the heating circuitry,e.g. LED indicators and the microcontroller, may be less than around 0.1W and in some examples may be less than around 0.01 W.

The example device 100 comprises a temperature sensing arrangement forsensing a temperature of the susceptor 132. For example, the temperaturesensing arrangement may comprise one or more temperature sensors, and inone example comprises one temperature sensor for each zone of thesusceptor 132. In one example, as described above, the susceptorcomprises a first zone 132 a and a second zone 132 b and the controller1001 operates the induction heating circuit 600 of the device 100 asdescribed above with reference to earlier figures to heat each zone by arespective inductor 124, 126.

In examples, the controller 1001 is configured to determine whether oneor more criteria that are indicative of a fault with the temperaturesensing arrangement are satisfied. If the controller determines thatsaid one or more predetermined criteria are satisfied then thecontroller 1001 takes a control action, for example stopping or reducingthe supply of energy to heat the susceptor 132 or issuing a warningsignal indicating that a fault has occurred. Examples of saidpredetermined criteria are described below. A safety feature maytherefore be provided which allows the controller 1001 to take a controlaction if a fault with the temperature sensing arrangement isidentified.

FIG. 17 shows further details of the susceptor 132. The susceptor 132,as described above, comprises a first zone 132 a, and a second zone 132b. A first thermocouple 133 a is arranged to measure the temperature ofthe first susceptor zone 132 a and a second thermocouple 133 b isarranged to measure the temperature of the second susceptor zone 132 b.As will be well understood, a thermocouple is a device used for sensingtemperature which comprises two dissimilar electrical conductors. Thetwo conductors are held at the same electric potential at a firstmeasurement end while a second end of the conductors is held at a knowntemperature to form a second reference end. In some examples, the endsof the two wires may be connected at the measurement end or in otherexamples the two wires may be connected to a single conductive surface,such as that of the susceptor 132. A voltage is generated between theconductors which is dependent on a temperature difference between themeasurement end and the reference end, according to the Seebeck effect.If the temperature of the reference end is known for example throughmeasurement by a temperature sensor, and in one example herein by use of10 kΩ thermistor, then a temperature at the measurement end can bedetermined from the voltage generated between the conductors.

In the example of FIG. 17, the first thermocouple 133 a comprises afirst pair of wires 1704, 1705 and the second thermocouple 133 bcomprises a second pair of wires 1708, 1709. In one example, boththermocouples are J-type thermocouples, that is, a first wire 1704, 1708of each pair is formed of constantan while a second wire 1705, 1709 ofeach pair is formed of iron. In other examples, different types ofthermocouples comprising different pairs of dissimilar conductors may beused, such as type E, K, M thermocouples for example, or a differenttype of thermocouple may be used for each susceptor zone 132 a, 132 b.

The first thermocouple 133 a comprises a first measurement junction 1706at which the constantan wire 1704 and iron wire 1705 of the firstthermocouple 133 a are joined together and which is attached to thefirst susceptor zone 132 a. In this example, the first measurementjunction 1706 is attached to the first susceptor zone 132 a byspot-welding to the surface of the susceptor 132. Similarly, the secondthermocouple 133 b comprises a second measurement junction 1310 at whichthe constantan wire 1708 and iron wire 1709 of the first thermocouple133 a are joined together and which is spot-welded to the susceptor 132at the second susceptor zone 132 b. Insulating sheaths 1707, 1711 covereach wire of the first thermocouple 133 a and second thermocouple 133 b.The first thermocouple 133 a terminates at a pair of terminals 1704 a,1705 b for providing reference end voltages for the wires 1704, 1705respectively to the controller 1001, allowing determination of thetemperature at the first measurement junction 1706 by the controller1001. Similarly, the second thermocouple 133 b terminates at a pair ofterminals 1708 a, 1709 a for providing reference end voltages to thecontroller 1001 for the wires 1708, 1709 respectively and allowingdetermination of the temperature at the second measurement junction 1710by the controller 1001. Each of the terminals 1704 a, 1705 a, 1708 a,1708 a may be connected to respective input pins of the controller 1001,or may be attached to the PCB 122 and thereby to the controller 1001.For example the terminals 1704 a, 1705 a, 1708 a, 1708 a may each besoldered to the PCB 122, to provide an input to the controller 1001.

Another example of a temperature sensing arrangement for the susceptor132 is shown in FIG. 18. In this example the temperature sensingarrangement again comprises two thermocouples each comprising aconstantan wire. However, in this example the two thermocouples share aniron wire. That is, a first thermocouple 183 a is for allowing thecontroller 1001 to determine a temperature of the first susceptor zone132 a and comprises a first constantan wire 1804 attached to the firstsusceptor zone 132 a at a measurement end 1806. An iron wire 1805 formsa second wire of the first thermocouple 183 a and is attached at thesecond susceptor zone 132 b in this example. A second thermocouple 183 bfor measuring the temperature of the second susceptor zone 132 bcomprises a first constantan wire 1808 attached to the second susceptorzone 132 b at a point 1810. The iron wire 1805 also forms the secondwire of the second thermocouple 132 b. The first constantan wire 1804 ofthe first thermocouple 132 a comprises a reference end 1804 a, the firstconstantan wire 1808 of the second thermocouple 132 b comprises areference end 1808 a, and the iron wire 1805 forming the second wire ofboth the first and second thermocouples 183 a, 183 b comprises areference end 1805 a. The reference ends 1804 a, 1805 a, 1808 a providerespective reference end voltages for each of the wires 1804, 1805, 1808to the controller 1001 to allow the controller 1001 to determinetemperatures of the first and second susceptor zones 132 a, 132 b. Thatis, the reference voltages provided by the constantan wire reference end1804 a of the first thermocouple 183 a and the reference voltageprovided by the iron wire reference end 1805 a allow the controller 1001to determine the temperature of the first susceptor zone 132 a.Similarly, the reference voltages provided by the constantan wirereference end 1808 a of the second thermocouple 183 b and the referencevoltage provided by the iron wire reference end 1805 a allow thecontroller 1001 to determine the temperature of the second susceptorzone 132 b. As described with reference to the example shown in FIG. 17,the junction ends 1804 a, 1805 a, 1808 a of the wires may be attached tothe PCB 122 to provide reference voltages to the controller 1001. In thearrangement shown in FIG. 18, the susceptor 132 is iron. This allows forthe use of the common iron reference wire 1805, since, because the ironwire 1805 and the susceptor 132 are made of the same material, the pointat which the iron wire 1805 joins to the susceptor 132 is not athermocouple junction. Providing three wires, as in the arrangementshown in FIG. 18, may be advantageous when compared to providing fourwires, as in the arrangement of FIG. 17, in that it requires fewer wiresto be attached, e.g. soldered, to the PCB 122.

FIG. 19 shows a schematic representation of apparatus 1900 for providingcontrol functions relating to temperatures in the device 100. As can beseen from FIG. 19, the apparatus 1900 is arranged to receive thereference voltages provided by the reference ends 1804 a, 1805 a, 1808 aof the thermocouple wires 1804, 1805, 1808 shown in the temperaturesensing arrangement example of FIG. 18. Describing first the input fromthe constantan wire 1804 of the first thermocouple 183 a, the constantwire reference end 1804 a of the first thermocouple 183 a is attached tothe apparatus 1300 at a point P8. From there, the reference voltagesignal at 1804 a is arranged to be supplied to the positive and negativeinput terminals of a first thermocouple op-amp U4A (component typeOPA2376). The point P8 is connected to the positive terminal of theop-amp U4A via a 2.49 kΩ resistor R26 and a 2.2 nF capacitor C20. Thepoint P8 is connected to the negative terminal of the op-amp U4A via theresistor R26 and a second 2.49 kΩ resistor R27. The op-amp U4A isconnected in a closed-loop configuration with 536 kΩ resistor R23 and a2.2 nF capacitor C19 arranged in parallel between the output and thenegative terminal of the op-amp U4A. The positive terminal of the op-ampU4A is also connected to ground. The op-amp U4A is powered by a 3.8Vsupply, which is provided by the controller 1001 and which in connectedto a power input of the op-amp via a 100Q resistor R17. The power inputof the op-amp U4A is connected to ground via a 10 nF capacitor C14.

The first thermocouple op-amp U4A is configured to receive a signal fromthe first thermocouple constantan wire 1804 at the point P8 and provideamplify this signal for outputting to the controller 1001. In thisexample, the op-amp U4A provides a gain of 107.63 to the input signalfrom thermocouple reference end 1804 a. This gain may be provided byselecting appropriate values for the resistors R26, R27 and R23 forexample. In selecting a gain for the op-amp U4A, the range oftemperatures to be measured by the thermocouple 183 a, the referencevoltage produced by a J-type at that range of temperatures and asuitable voltage range to be provided to the controller 1001 may betaken into account. In one example, a thermocouple reference voltage of23.228 mV corresponds to a temperature reading of 425° C. To amplifythis reference voltage to a value of 2.5V to provide to the controller1001 requires a gain of 107.63, as provided by the example componentvalues shown in FIG. 19. An amplified first thermocouple voltage TC1 isprovided to the controller 1001 at a point 1901 which is connected tothe output of the op-amp U4A via a 1 kΩ resistor R22. The point 1901 isconnected to ground via a 100 nF capacitor C21. From the voltage signalTC1 provided at the point 1901 the controller 1001 is configured todetermine a temperature value measured by the first thermocouple 183 a,corresponding to the temperature of first susceptor zone 132 a.

The second thermocouple 183 b is, as described above, also a J-typethermocouple and provides a second thermocouple reference voltage TC2 tothe controller 1001 at a point 1902 in the same manner as the firstthermocouple reference voltage TC1 is provided to the controller 1001.This allows the controller 1001 to determine a temperature for thesecond susceptor zone 132 b in the same manner as described for thefirst thermocouple 183 b. It will be understood that a plurality ofresistors R37, R38, R39, R40 and capacitors C30, C31, C32 connected tothe second op-amp U4B provide the equivalent function as the resistorsR22, R23, R26, R27 and capacitors C19, C20, C21 described above inconnection with the first thermocouple op-amp U4A.

In addition to providing to the controller 1001 the amplifiedthermocouple voltages TC1, TC2, the apparatus 1900 is also configured toprovide a safety feature in the event that a temperature measured byeither of the thermocouples 183 a, 183 b exceeds a given threshold. Thatis, the output of the first thermocouple op-amp U4A is connected to anegative terminal input of a first precision comparator U3A (componenttype AS393). A positive input of the precision comparator U3A isconnected to a 2.5V signal via a 3.3 kΩ resistor R42 and to ground via a10 kΩ resistor R43. The precision comparator U3A is configured such thatif the amplified thermocouple voltage received from the op-amp U4Aindicates a temperature exceeding a particular value, the firstprecision comparator U3A outputs a stop signal/FF RST to stop heating ofthe susceptor 132 by the induction heating circuit 600. In examples, thesignal/FF RST is sent to the flip-flop 622 input RST, to reset theoutput value Q of the flip-flop 622, in the manner described above withreference to earlier figures, to thereby stop heating by the circuit600. In one example the threshold temperature value at which theapparatus 1900 is configured to send the signal to stop heating by thecircuit 600 is 280° C.

A second precision comparator U3B performs an equivalent function forthe second thermocouple 183 b, receiving an input at its negativeterminal from the second thermocouple op-amp U4B and being configured tosend the signal/FF RST if the received signal from the op-amp U4Bindicates that the second thermocouple 183 b is measuring a temperatureexceeding the threshold value.

A third comparator U6 is also included in FIG. 19 and provides foranother mechanism by which the signal/FF RST may be sent to stop heatingof the susceptor 132. The comparator U6 receives a signal COIL TEMP froma thermistor (not shown) located on the insulating member 128 whichindicates a temperature at the outside of the insulating member 128,proximate the coils 124, 126. The comparator U6 is configured to comparethis signal COIL TEMP to a threshold value and provide a signal, in thisexample a signal/FF RST to the flip-flop 622 to stop operation of thecircuit 600 to heat the susceptor 132 if the comparator U6 determinesthat the value of the temperature represented by COIL TEMP is too high.

It will be appreciated that for accurate measurement of the temperaturesof the susceptor zones 132 a, 132 b, each measurement ends (1706, 1710in the example of FIG. 17, or 1806, 1810, 1805 in the example of FIG.18) should be at a temperature which is substantially the same as therespective temperature of the zone 132 a, 132 b for which thetemperature is being measured. As such, good thermal conductivitybetween the susceptor 132 and the thermocouples 183 a, 183 b should bemaintained.

To describe this with reference to an example, it will be appreciatedthat if, for example, the attachment point 1806 of the first constantanwire 1804 were to become detached from the first susceptor zone 132 athen thermal conduction between that susceptor zone 132 a and the firstthermocouple 183 a would be lost or at least significantly reduced. Insuch a circumstance, the first thermocouple 183 a may, for example,remain detached from but within close proximity of the susceptor 132(and may continue to be heated by the susceptor 132 to some extent bye.g. radiative or convective processes). However, since heat would nolonger be properly conducted between the susceptor 132 and the detachedfirst thermocouple 183 a, the temperature of the first susceptor zone132 a (which the first thermocouple 183 a is intended to measure) andthe detached thermocouple wire 1804 may diverge as the first susceptorzone 132 a is heated. In this circumstance, the first thermocouple 183 awould not, for example, allow the controller 1001 to accuratelydetermine increasing temperature of its corresponding first susceptorzone 132 a as said zone 132 a is heated. If heating of the firstsusceptor zone 132 a were to continue in this manner without itstemperature being properly measured by use of the first thermocouple 183a, a danger of overheating of the susceptor 132 may arise. It will beappreciated that the same principle described above also applies to thesecond susceptor zone 132 b in the circumstance that the secondthermocouple 183 b were to become decoupled therefrom.

In one example, the device 100 is configured to cut off heating of thesusceptor 132 in the event of an indication that either of the susceptorthermocouples 183 a, 183 b is not correctly measuring the temperature ofthe susceptor 132. This may be advantageous in that a safety feature isprovided which allows cutting off of power to heat the susceptor 132when the controller 1001 determines that the temperature of thesusceptor 132 is not being properly measured.

In one example, as described above in relation to the induction heatingcircuit 600 of the device 100, a power supplied to the induction heatingcircuit for heating of the susceptor 132 may be determined by measuringthe DC voltage supplied by the DC supply and the DC current supplied bythe DC supply and calculating a product of these values to obtain apower. From this measurement of power, the controller 1001 may beconfigured to determine an amount of energy supplied to the circuit overa given period of time. As above, the controller 1001 is also configuredto receive temperature measurements from the thermocouples 183 a, 183 b.The controller 1001 is configured to determine a change in temperaturemeasured by the thermocouples 183 a, 183 b over the given period oftime. In one example then, the controller 1001 may determine the amountof energy supplied per 1° C. rise in temperature measured by one of thesusceptor thermocouples 183 a, 183 b. For example, the controller 1001may determine a ratio of the amount of energy supplied to the firstsusceptor zone 132 a over a given time period to the increase in thetemperature measured by the first thermocouple 183 a over that timeperiod. Similarly, the controller 1001 may determine the amount ofenergy supplied to the second susceptor zone 132 b per 1° C. rise intemperature measured by the second thermocouple 183 b—in examples thisamount of energy per ° C. rise is measured temperature may differ tothat for the first zone due to differing properties of the first andsecond inductors 124, 126.

In one example, during setup of the device 100 for example, the amountof energy supplied to a given one of the inductors 124, 126 for heatingthe corresponding susceptor zone 132 a, 132 b of that inductor whichresults in a measured temperature rise by the corresponding thermocouple183 a, 183 b of a given amount may be determined. That is, throughtesting or otherwise, it may be determined that in normal operationwhen, for example, the first thermocouple 183 a is properly attached tothe first susceptor zone 132 a, on average, supplying 400-1000 mJ, oraround 500 mJ to the first inductor 124 results in a temperature risemeasured by the first thermocouple 183 a of 1° C. That is, the ratio ofenergy supplied to measured increase in temperature may be 400-1000 mJ/°C., or around 500 mJ/° C. In another example, it may be determined thatat any point during normal operation of the device, the maximum value ofthe ratio of energy supplied to the first inductor 124 to rise intemperature measured by the first thermocouple 183 a is 500 mJ/° C.

However, if, as described above, the first thermocouple 183 a becomesthermally decoupled from the first susceptor zone 132 a, then the amountof energy supplied to the first inductor 124 which results in a 1° C.temperature rise being measured by the first thermocouple 183 a maysignificantly increase. For example, through testing it may be foundthat if the first thermocouple 183 a is decoupled from the firstsusceptor zone 132 a then the ratio of the energy, as measured in mJ,supplied to the first inductor 124 to the increase in temperaturemeasured by the first thermocouple 183 a may be around a very highnumber, such as 40,000 to 100,000 J/° C. That is, if the firstthermocouple 183 a becomes detached from the susceptor zone 132 a and isno longer properly measuring its temperature, then the thermocouple mayrecord only small increases in temperature, despite the susceptor 132 abeing heated by the inductor 124. The controller 1001 may thus determinethat if the ratio of energy supplied to heat the first susceptor zone132 a to the temperature rise measured by the first thermocouple 183 ais greater than a given amount, then the first thermocouple 183 a may nolonger be correctly measuring the temperature of the first susceptorzone 132 a, e.g. the first thermocouple 183 a may have become detachedfrom the susceptor 132. Similarly, the controller 1001 may determinethat if a ratio of energy supplied to heat the second susceptor zone 132b to the temperature rise measured by the second thermocouple 183 b isgreater than a given amount, then the second thermocouple 183 b may nolonger be correctly measuring the temperature of the second susceptorzone 132 b.

In this example, the controller 1001 is configured to stop the supply ofpower to heat the susceptor 132 if the ratio of energy supplied to heatthe susceptor 132 to the temperature rise measured by one of the firstand second thermocouples 183 a, 183 b is greater than a predeterminedamount. Cutting-off of the power for heating the susceptor 132 in thismanner may act as a safety feature which helps prevent over-heating ofthe susceptor 132 where one (or both) of the thermocouples 183 a, 183 bhas become detached from the susceptor 132 or is otherwise no longerproviding accurate susceptor temperature measurements. The controller1001 may, for example, stop the supply of energy to heat the susceptor132 by, deactivating the circuit 600 by use of a signal sent to thecontrol arrangement of the induction heating circuit 600. In anotherexample, the controller 1001 may take another control action in responseto determining that the ratio is greater than the predetermined amount.For example, the controller 1001 may reduce the power supplied to thecircuit 600 to heat the susceptor 132. For example, the controller 1001may reduce the control voltage 1031 to reduce power supplied to heat thesusceptor 132.

In one example, an expected measured value of the ratio of energy toheat one of the susceptor zones 132 a, 132 b to the temperature risemeasured by the corresponding thermocouple 183 a, 183 b in normaloperation may be around 500 mJ/° C. and the controller 1001 may beconfigured to stop the supply of power to heat the susceptor 132 if itdetermines that the ratio has become greater than or equal to a examplevalue from 2000 mJ/° C. to 4000 mJ/° C. As such, in this example, amargin, in this case of around 1500 mJ/° C. to 3500 mJ/° C. above theexpected value is provided which may allow for minor increases of theratio above the expected level to occur without the controller 1001taking a control action such as cutting off the power supply.

In examples, during operation of the device 100 a value for the ratiobetween the energy supplied to heat the susceptor 132 to the measuredincrease in temperature sensed by one of the thermocouples 183 a, 183 bmay be determined at predetermined intervals. The predeterminedintervals may be, for example, the same as those used for controllingthe induction heating circuit 600, such as those described above, andmay in one example be of length 1/64 s. Where one of the thermocouples183 a, 183 b has become detached from the susceptor 132, it has beenfound that in one example a ratio of 4000 mJ/° C. for that thermocoupleis exceeded within around 0.5 s. As such, in this example where thecut-off ratio is set at 4000 mJ/° C., the supply of power may be cut-offwithin around 0.5 s, and thus substantial overheating of the susceptor132 may be avoided.

FIG. 20 shows a flowchart representation of an example method 1400 ofcontrolling the device 100. In an example the method 1400 is performedby the controller 1001. At block 1402 power is supplied from the powersupply to heat the susceptor 132. At block 1404 an energy ΔE supplied tothe heating circuit to heat the susceptor 132 over a given time periodis determined. As described above, at any one time, power is onlysupplied to one or the other of the first inductor 124 and the secondinductor 126 during the given time period. As such, the power measuredat block 1404 is a power supplied to the one of the first inductor 124and second inductor 126 which is active during the given time period.This may be determined as described above, for example, by measuring aDC voltage and a DC current supplied to the heating circuit by the DCsupply and calculating a product of these values. For example, an energyused over the time period may be determined by determining an averagepower over the time period and multiplying this by the duration of thetime period, e.g. of 1/64 s.

At block 1406 a change in temperature ΔT measured by one of thesusceptor thermocouples 183 a, 183 b over the time period is determined.The temperature change ΔT will in examples correspond to the temperaturechange measured by the thermocouple corresponding to the inductor towhich power was supplied during the given period. That is, for example,if during the given time period power is supplied to the first inductor124, then the energy ΔE is the energy supplied to the first inductor 124during the given time period and the temperature change ΔT is thetemperature change recorded by the first thermocouple 183 a during thegiven time period. At block 1408 a ratio ΔE/AT of the energy ΔE suppliedover the time period to the change in the temperature ΔT measured by therespective susceptor thermocouple 133 a, 133 b over the given timeperiod is determined. At block 1410 the ratio ΔE/ΔT is compared to acut-off value. The cut-off value is a threshold value for ΔE/ΔT whichmay be predetermined during setup of the device 100 as discussed above.The cut-off value may be greater than an expected value for the ratioΔE/ΔT when the device 100 is operating normally. The cut-off value maybe greater than the expected value by a given margin, e.g. a margin ofaround 500 mJ/° C., or around 1000 mJ/° C.

If, at block 1410 the controller 1001 determines that the determinedvalue for the ratio ΔE/ΔT is less than the cut-off value then the methodreturns to block 1402 and the controller 1001 continues to allow powerto be supplied to heat the susceptor 132 for another instance of thegiven time period. If, however, at block 1410 the controller 1001determines that the ratio ΔE/ΔT for the given time period is greaterthan or equal to the cut-off value then the method proceeds to block1412.

At block 1412 the controller 1001 determines if the inductor 124, 126has been active to heat the susceptor 132 for a predetermined length oftime, of for example 0.2-1 s or around 0.5 s. The predetermined lengthof time may, for example comprise a number of consecutive predeterminednumber of time periods. For example, at block 1412 the controller 1001may determine if the circuit 600 has been active to heat the susceptor132 for 32 consecutive time periods of 1/64 s, totaling a length of timeof 0.5 s. If the answer is “no” then despite the determined ratio ΔE/ΔTat block 1410 being above the cut-off value the method returns to block1402 and the controller 1001 continues to allow heating of the susceptor132. The step performed at block 1412 allows the method 1400 to takeinto account that when the controller 1001 is operating to maintain thetemperature of e.g. the first susceptor zone 132 a, rather than actingto increase the temperature of said zone 132 a, energy may be suppliedto heat the zone 132 a while relatively small increases in temperatureare recorded by the first thermocouple 183 a. Therefore, a value forΔE/ΔT above the cut-off value may be obtained which does not necessarilyindicate a fault with the operation of the thermocouple 183 a to measurethe temperature of the first susceptor zone 132 a.

In another example, the predetermined period over which ΔT and ΔE arecalculated may be the same as the length of time used at block 1412.That is, in one example, ΔT is the change of measured temperature over a0.5 s period and ΔE the energy supplied over that 0.5 s period. In thisexample, therefore, at block 1412 the controller 1001 determines if thecircuit 600 has been active to heat the susceptor 132 for the entiretyof the previous 0.5 s time interval and if the answer is “yes” thecontroller 1001 proceeds to block 1414. In this example, it will beunderstood that during the 0.5 s period the controller 1001 maydetermine once every 1/64 s which of the inductors 124, 126 if eithershould be active to heat the susceptor 132 and thus block 1412 mayamount to the controller 1001 determining that one of the inductors 124,126 was active for each of the 32 previous 1/64 s intervals.

At block 1414, the controller 1001 cuts of the supply of power to heatthe susceptor 132. This acts as a safety mechanism which may preventover-heating of the susceptor 132 in the event that one or both of thethermocouples 183 a, 183 b is not properly measuring changes intemperature of the susceptor 132 for a given length of time.

As described in examples above, the controller 1001 is configured toimplement the described safety feature by determining if a predeterminedcriteria is satisfied based on a temperature of the susceptor 132 andthe energy supplied to the circuit 600. However, it will be appreciatedthat in other examples, the controller 1001 may determine whether totake a control action such as stopping the supply of energy to theheating circuit based on one or more predetermined criteria using adifferent temperature measured in the device 100. For example, theabove-described method of determining, based on a comparison between ameasured increase in the temperature and an amount of energy supplied tothe heating circuit, if a criteria is met which indicates a fault inmeasuring a temperature in the device 100 may be used elsewhere in thedevice 100.

As described above, the controller 1001 of the example induction heatingdevice 100 is configured to perform a method of stopping supply ofenergy to heat a susceptor if one or more predetermined criteria thatare indicative of a fault with measurement of a temperature of thesusceptor are satisfied. However, in other examples, the above-describedmethod may be applied by a controller in a different type of aerosolgenerating device than a device which comprises an inductive heatingcircuit. For example, the above-described example method may be appliedin an aerosol generating device comprising a heating circuit which isnot an inductive heating circuit. In such an example the heating circuitmay comprise one or more resistive heating elements for generating heatto heat an aerosol generating material when a current is passed throughsaid resistive heating elements. In an example a temperature sensingarrangement may comprise temperature sensors for sensing temperatures ofone or more such a resistive heating elements and such temperaturesensors may be attached to or in close proximity to said heatingelements during normal use. The controller of such a device may beconfigured to determine a fault with the arrangement, such as detachingof one of the temperature sensors from the heating element, by methodsdescribed above with reference to FIG. 20.

Examples above have described the controller 1001 determining apredetermined criteria which is dependent on a measured increase intemperature of the thermocouples 183 a, 183 b. In other examples, thecontroller 1001 may, for example, determine that a fault has occurredwith the temperature sensing of the susceptor 132 by the thermocouples183 a, 183 b if the controller determines that energy has been suppliedto heat the susceptor 132 over a given period while a reduction intemperature has been measured by the thermocouples 183 a, 183 b overthat same period.

Certain methods described herein may be implemented by way ofnon-transitory computer program code that is storable on anon-transitory storage medium. For example, in certain examples, thecontroller 1001 may comprise a non-transitory computer readable storagemedium comprising a set of computer-readable instructions stored thereonand a processor to perform a method described herein when executed bythe controller 1001. The controller 1001 may comprise one or moreprocessors. For example, in some examples, as described above, thecontroller 1001 is a programmable micro-processing unit. The controller1001 may comprise a storage medium comprising a set of machine readableinstructions, e.g. in the form of computer code, which when executed bythe controller 1001 cause a method described herein to be performed.

While some of the example circuits described herein make use of siliconFETs for certain switching functions other suitable components may beused in place of such FETs. For example, components comprising widebandgap materials such as silicon carbide, SiC, or gallium nitride, GaN,may be used. Such components may in some examples be FETs but in otherexamples may be high electron mobility transistors (HEMT). Suchcomponents may be faster and have higher breakdown voltages than siliconFETs which may be advantageous in some examples.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

1. An apparatus for an aerosol generating device, the apparatuscomprising: a heating circuit for heating a heating arrangement, theheating arrangement arranged to heat an aerosol generating material tothereby generate an aerosol; a temperature sensing arrangement forsensing a temperature of the device; and a controller for controlling asupply of energy to the heating circuit, wherein the controller isconfigured to: determine a characteristic that is indicative that energyis being supplied to the heating circuit over a given time period, anddetermine a change in temperature sensed by the temperature sensingarrangement over the given time period; and take a control action if,based on the determined characteristic and the increase in temperaturesensed by the temperature sensing arrangement over the given period, thecontroller determines that one or more predetermined criteria that areindicative of a fault with the temperature sensing arrangement aresatisfied.
 2. The apparatus of claim 1 wherein the temperature sensingarrangement comprises a temperature sensor for attaching to the heatingelement to sense a temperature of the heating element, and the one ormore predetermined criteria are indicative that the temperature sensoris detached from the heating element.
 3. The apparatus of claim 1,wherein the controller is configured to: determine a ratio of the amountof energy supplied to the heating circuit over the given period to theincrease in temperature sensed by the temperature sensing arrangementover the given period; and take the control action if the supply ofenergy to the heating circuit if the ratio equals or exceeds apredetermined value.
 4. The apparatus of claim 4, wherein thepredetermined value for the ratio is 2000 mJ/° C. to 6000 mJ/° C., oraround 4000 mJ/° C.
 5. The apparatus of claim 1, wherein the controlaction taken by the controller if the one or more predetermined criteriaare satisfied is to adjust the supply of energy to the heating circuit.6. The apparatus of claim 5, wherein the control action taken by thecontroller if the one or more predetermined criteria are satisfied is tostop the supply of energy to the heating circuit.
 7. The apparatus ofclaim 5, wherein the control action taken by the controller if the oneor more predetermined criteria are satisfied is to reduce the supply ofenergy to the heating circuit.
 8. The apparatus of claim 1, wherein thecontroller is configured to determine the predetermined criteria for thegiven time period and to determine the predetermined criteria once foreach of one or more further predetermined periods in a usage session ofthe device, wherein, optionally, the predetermined periods are each of aduration of 1/80 s to 1/20 s or of a duration of around 1/64 s.
 9. Theapparatus of claim 1, wherein the heating circuit is an inductionheating circuit, and the heating element is a susceptor arrangement forbeing inductively heated by the induction heating circuit, and whereinthe temperature sensing arrangement comprises a temperature sensor forsensing a temperature of the susceptor arrangement.
 10. The apparatus ofclaim 9, wherein the temperature sensor is a thermocouple for attachingto the susceptor arrangement.
 11. The apparatus of claim 1, wherein thetemperature sensing arrangement comprises: a first temperature sensorfor measuring a first temperature in the device; and a secondtemperature sensor for measuring a second temperature in the device; andwherein the increase in temperature sensed by the temperature sensingarrangement over the given period is an increase in the firsttemperature or an increase in the second temperature.
 12. The apparatusof claim 11, wherein the first temperature is a temperature of a firstheating zone in the device and the second temperature is a temperatureof a second heating zone in the device.
 13. The apparatus of claim 12,wherein the heating circuit is configured to selectively heat the firstheating zone and the second heating zone, and the controller isconfigured to, during the given period, activate the heating circuit toheat only one of the first heating zone and the second heating zone. 14.The apparatus of claim 13, wherein the controller is configured todetermine the predetermined criteria for the given time period and todetermine the predetermined criteria once for each of one or morefurther predetermined periods in a usage session of the device andwherein during each period the heating circuit is configured toselectively heat only one of the first heating zone and the secondheating zone.
 15. The apparatus of claim 14 wherein the increase intemperature sensed by the temperature sensing arrangement and used todetermine the one or more criteria for each period during the usagesession is: an increase in the first temperature if the heating circuitis active to heat the first heating zone during the period; and anincrease in the second temperature if the heating circuit is active toheat the second heating zone during the period.
 16. The apparatus ofclaim 12, wherein: the heating circuit is an induction heating circuitcomprising a first inductor coil and a second inductor coil; the heatingelement is a susceptor arrangement; the first heating zone is a firstzone of the susceptor arrangement arranged in use to be heated by thefirst inductor coil; and the second heating zone is a second zone of thesusceptor arrangement arranged in use to be heated by the secondinductor coil.
 17. The apparatus of claim 16, wherein the firsttemperature sensor is a first thermocouple for attaching to the firstzone of the susceptor arrangement and the second temperature sensor is asecond thermocouple for attaching to the second zone of the susceptorarrangement.
 18. The apparatus of claim 17, wherein the firstthermocouple and second thermocouple are J-type thermocouples eachcomprising a constantan wire and an iron wire.
 19. The apparatus ofclaim 18, wherein the first thermocouple comprises a first constantanwire and the second thermocouple comprises a second constantan wire andthe first thermocouple and second thermocouple share a single iron wire.20. An aerosol generating device comprising an apparatus, the apparatuscomprising: a heating circuit for heating a heating arrangement, theheating arrangement arranged to heat an aerosol generating material tothereby generate an aerosol; a temperature sensing arrangement forsensing a temperature of the device; and a controller for controlling asupply of energy to the heating circuit, wherein the controller isconfigured to: determine a characteristic that is indicative that energyis being supplied to the heating circuit over a given time period, anddetermine a change in temperature sensed by the temperature sensingarrangement over the given time period, and take a control action if,based on the determined characteristic and the increase in temperaturesensed by the temperature sensing arrangement over the given period, thecontroller determines that one or more predetermined criteria that areindicative of a fault with the temperature sensing arrangement aresatisfied; wherein the aerosol generating device is for generating anaerosol for being inhaled by a user.
 21. An aerosol generating deviceaccording to claim 20 wherein the aerosol generating device is a tobaccoheating device, also known as a heat-not-burn device.
 22. An aerosolgenerating system comprising an aerosol generating device according toclaim 20 and an article comprising an aerosol generating material forbeing heated by the device in use to thereby generate an aerosol.
 23. Anaerosol generating system according to claim 22, wherein the aerosolgenerating device is a tobacco heating device, also known as aheat-not-burn device, and wherein the aerosol generating materialcomprises a tobacco material for being heated by the device in use. 24.A method for a controller of an apparatus for an aerosol generatingdevice, the apparatus comprising: a heating circuit for heating aheating arrangement, the heating arrangement arranged to heat an aerosolgenerating material to thereby generate an aerosol; a temperaturesensing arrangement for sensing a temperature of the device; and thecontroller, wherein the controller is for controlling a supply of energyto the heating circuit; wherein the method comprises: determining acharacteristic that is indicative that energy is being supplied to theheating circuit during a given time period; determining a change intemperature sensed by the temperature sensing arrangement over the giventime period; and taking a control action if, based on the characteristicand the increase in temperature sensed by the temperature sensingarrangement over the given period, the controller determines that one ormore predetermined criteria that are indicative of a fault with thetemperature sensing arrangement are satisfied.
 25. A non-transitorycomputer-readable medium having stored thereon a machine-readableinstructions which when executed by a processor cause the methodaccording to claim 24 to be performed by the processor.
 26. (canceled)